Okay, I've seen many comments - particularly from WinterSEO and other people - about how "bloated" or overly complex the new Adeptus Mechanicus codex is. Layers and layers of buffs, and all that complaints about it being needlessly complicated, something that frightens off new players, the usual "rules bloat" complaints, and even the "overpowered" or "codex creep" nonsense. Even Goonhammer complains about how they took the violent way to write the codex instead of the easy or difficult way.
I'm here to set the record straight. The new Adeptus Mechanicus codex isn't actually that bloated or complex. What it does, however, is offer you tons of new options. The problem is when people try to remember and use all those options, and then they find it just so overwhelming.
My advice? Don't be greedy. Don't min-max. Keep it simple. You do NOT need to know each and every option. You do NOT need to use every option. So let's break down the codex into something simpler.
First, the Canticles of the Omnissiah and the Doctrina Imperatives. There is a simple way to understand them. Skitarii units - that is, almost every unit in the book that are the more conventional tanks, troops or infantry (that is to say, the very human-looking cyborgs that sometimes have wings), cavalry and walkers EXCEPT Robots - get Doctrina Imperatives. Only Cult Mechanicus units get Canticles of the Omnissiah, and these will be all your priests (Electro-priest, Tech-priests and all HQ units except the Skitarii Marshal, easy to remember because the Marshal has Skitarii in his name, eh?), Servitors (both normal and Kataphron), Robots and Cybernetica Datasmith (in fact, Cybernetica Datasmiths are also Tech-priests - they have the Tech-priest Keyword!), unless you play Mars. That's all, really. Servitor? Cult Mechanicus. Robots and their accompanying Datasmith? Cult Mechanicus. Priests in the name? Cult Mechanicus (duh). Everything else is Skitarii.
Secondly, Holy Orders. You don't need all of them. You don't want all of them. Choose one or two, if you fancy any of them, but you don't need to remember all of them. Also, they only affect one Core unit, so yeah. Too complex and too many for you? Choose one, choose your favorite, and remember that it only affects one Core unit close to that Tech-priest you upgraded. That's all. What is your favorite? If you play a Robot-heavy list, keep the Tech-priest close to your Kastelan Robots, keep the Robots close to your Cybernetica Datasmith (just think of the Cybernetica Datasmith and Kastelan Robots as one unit, don't bother with switching Protocols - rely on the Stratagem Binharic Override to do so instead, but keep in mind their different Movement Characteristics and Wargear - so just always put your Datasmith in front of your Robots so that he can keep up with them). Most of the times, you'll probably just want the Logi for Robots because they nullify AP -1 and AP -2, useful for Kastelan Robots on Aegis Protocol. I mean, you could improve the Strength of their Ranged weapons or let them explode on 6s to hit for shooting (though your Tech-priest will need to stand still and switch to the Advanced Part for an entire turn, but then again, dakkabots will probably stay in your backline). Again, my advice is don't be greedy and try to overbuff them. Strength 7 phosphor weapons aren't all that useful, to be honest, you might as well keep the Artisans with infantry instead. And don't bother changing the Protocol. Or maybe you like Artisans, in which case you'll probably put them next to Skitarii Rangers or Vanguard to buff the Strength of their guns, and therefore you want to activate the Advanced part. Or if you love Dakka, Magi is for you, especially the Advanced part. Remember, Advanced part takes a turn to activate, so just take note that your Tech-priest is inactive for a turn just to switch. Shouldn't be too hard. If you really need a reminder, use a marker or different-colored dice to mark him as switching Protocols, then flip the dice (1 for Initial Part, 2 or 6 for Advanced Part) or marker to indicate that he's now advanced for the rest of the battle.
Thirdly, focus on what you want to build your army around. You want to build around hordes of infantry? Then focus on Skitarii as a horde army. This will probably revolve around the whole Enriched Rounds or Galvanic Volleys Fire Stratagems, but remember you can only use it on one unit each. Yeah, there will be people who min-max and want to buff them as much as possible, so they try to do too much, like throw in a Tech-priest Manipulus in it, give him the Artisan or Magi Holy Order (usually they will just give him Artisan), and then buff the Skitarii Unit's core weapons further. But that's just for one unit. You're playing a horde army. You have plenty of other squads and models. You don't need to remember the buffs for all of them. Just think of a single super-unit next to your Tech-priest Manipulus, and then play the others normally. Really, that's about it. Of course, if you want, you can bring a Skitarii Marshal to either further buff the shooting with re-roll auras, or you can give him the Warlord Trait that allows a single Skitarii unit to perform Actions while shooting, or you can just not bother with that because you're playing a horde army, after all. If you can't be bothered with that Warlord Trait, then good - you don't need 6 blocks of 20 Skitarii Rangers or Vanguard. Keep in mind that you don't necessarily have to play horde. And as much as people whine about how "overpowered" they are with the new Stratagems, remember that Skitarii are Toughness 3 with a 4+ armor save and 6++ invulnerable save. Even with Bulwark Imperative (which halves their movement), they are still going to get shot off the board by bolter fire (especially AP -2 bolt rifles on Tactical Doctrine). They are not going to survive getting into range with their 18" radium carbines or 15" for rapid fire galvanic rifles (Bulwark Imperative might improve their survivability, but halves their movement, so there's a tradeoff). Any smart opponent is going to shoot them off the board, and they will be susceptible to blast weapons, heavy stubbers and anti-infantry weapon that will easily mow them off the board. And unless you take Graia (which I doubt), you'll be susceptible to morale as well, though you do have a Stratagem to mitigate that (as long as you're near an Objective), but chances are, you will likely never be able to make use of your super-squad combo with those Stratagems unless you keep them out of line of sight (though your enemy might have weapons that ignore line of sight, so that point is moot) and hold them back for your opponent to slowly approach them.
So the combos and super-units of 20 Vanguard and Rangers might sound powerful, but practically wise, they are hard to pull them off unless your opponent decides to ignore them for some weird reason. You can plan and come up with whatever combos you want, but no battle plan ever survives enemy contact. That is not to say you should dismiss the Stratagems and not play horde armies - quite the contrary - but don't expect to have this ideal scenario where your opponent walks a Knight or a horde within 15" of your 20-strong Skitarii Rangers so that you can conveniently pop off Galvanic Volley Fire for maximum carnage. More often than not, it won't happen, and you'll most likely have a few of them dead by the time you are within rapid-fire range. Keep that in mind and plan to mitigate the damage, improvise and adapt. Don't be afraid to pop off Galvanic Volley Fire even if you only have 10 Skitarii Rangers left, or Enriched Rounds with 10 Skitarii Vanguard (that's still 30 shots, with 15 auto-wounds). The damage output will still take enemies by surprise and it can prove to be the difference between victory and defeat. Just don't be rigid and think that you need 20 dudes for the Stratagems and min-max. You don't. Putting 10 Skitarii Rangers or Vanguard in a Skorpius Dunerider, zooming them up the table, disembarking them and popping off the Stratagem can still deal an incredible amount of damage, plus your Skitarii Rangers or Vanguard will have a higher chance of surviving than in the open or inside a building (if they can all fit inside a building, but that won't protect them from weapons that ignore line of sight). Of course, if they wreck your Skorpius Dunerider and kill the poor Skitarii Rangers or Vanguard inside (which probably means they aren't targeting your other, more important units), then well...the Stratagem is a nice plus, but it's not entirely necessary. Plus it's easier to have mutiple 10-man squads ferried around in a Skorpius Dunerider than trying to hide 20-men squads all over the board.
But yeah...notice something? If you play an infantry-heavy horde army, then why do you need to remember so many things? The answer is...you don't. Just particular combos here and there. Maybe you'll want to add in Ironstrider Ballistarii for anti-tank. Cool. But you won't be buffing them (at least I don't think you will). Canticles? What's that? Unless you play Mars, you probably don't need to bother. And since this is a shooting army, the only Canticles that apply to your Skitarii horde army will probably be Shroudpsalm and Benediction of the Omnissiah - you will most likely avoid having your Skitarii in close combat anyway. I mean, you can take the rest for your Cavalry or Sydonian Dragoons or Sicarian assassins if you want - Chant of the Remorseless Fist will be great for them, but by and large they won't really affect the rest of your Skitarii dudes (and you might as well get the permanent +1 to wound roll in melee from Ryza than a single turn of +1 Strength, anyway). If you don't want to remember the Canticles, then don't use Mars, keep it simple. Use maybe Lucius or Stygies VIII to make your dudes more durable. Besides, Mars is better for multiple smaller units, given that you get re-rolls for each unit, so you'll want Mars if you're running 5-man Skitarii squads with special weapons instead. If you do that, obviously you don't have to bother with all the buffs, super-unit, Stratagems and absurd combos, and just focus on re-rolling hit rolls for your Skitarii, Benediction of Omnissiah for one turn, and then maybe select a specialized unit next to a Tech-priest with the Martian Warlord Trait to benefit from the Benediction of the Omnissiah (remember, only one unit can benefit from it).
If you want to go Robot-heavy or tank-heavy, you can too. There's a Forge World for that! Datahoard Forge World (Magnabonded Alloys) with Autosavant Spirits. You would want a Tech-priest Enginseer to buff your tanks to BS 2+, while a Tech-priest Dominus accompanies a single unit of Kastelan Robots and a Cybernetica Datasmith. That will make them a lot tougher. Oh, and throw in a couple of Archaeopters if you want, for more firepower. Or if you want to go Walker heavy, that's fine too! You can build around huge units of Ironstrider Ballistarii and Sydonian Dragoons and maybe even the Onager Dunecrawlers, Skorpius Duneriders or Skorpius Disintegrators, and Archaeopters, and then use Datahoard Forge World (Magnabonded Alloys) with Servo-focused Auguries to buff their Cognis weapons. Do you need to buff them with Characters, auras and stuff? No. Keep it simple, your Forge World stuff already buffs them. You can keep in mind the Dunestrider or Crushing Weight Stratagems (also cool for Kastelan Robots), but that's about it. Do you need to remember Galvanic Volley Fire, Enriched Rounds or the Skitarii infantry buffs? Nope. You sure don't have to. Don't be greedy, keep it simple.
Find a theme that you like, and build around it. If you like melee, then pick Ryza or an Expansionist Forge World. Ryza might be better in melee because of the bonus to wound and to charge rolls, but if you have the Rugged Explorators paired with Accelerated Actuators, you can have very mobile Skitarii Ranger squads moving around in Skorpius Duneriders or not, and perhaps multiple small squads will be better for them, so you don't really have to bother with all those "layers of buffs" unless you're that desperate to buff a single 5-man squad to...I don't know. Still, it's not like you can't - the Genetor Holy Order will give your dudes a boost, and for practical reasons, while you would certainly love to boost your Serberys cavalry in combat, your Tech-priest will most likely be accompanying Rangers or Vanguard instead. So it really depends on your build.
There's another option to keep everything simple. Can't stand the "complexity" of remembering your Canticles and Doctrina Imperatives? Turn them off. Take a single Super-heavy Detachment of Imperial Knights (like me!). Ally in a small Patrol Detachment to your Knights and have the infantry perform Actions and capture Objectives while your Knights move around. Are Canticles and Doctrina Imperatives necessary? No. They are powerful in the right circumstances, and definitely useful, but the sheer firepower of your Knights should more than make up for not having them. After all, if most of your points are going into Knights, you'll hardly need the +1 to your Ballistic Skill for a 20-men Skitarii Ranger block. Nor will you bother with buffing the Strength and Weapon Skill of your Skitarii or Robots or walkers because your Knights will be doing the heavy lifting in melee - in this case you will want your Skitarii to hang back in the backline and capture Objectives (same goes for Kataphron Servitors and tanks and other units). This applies to other allies such as Imperial Guard as well.
So yeah, the best advice is to keep it simple. If you feel overwhelmed by all the options in the codex, remember that you do NOT have to use all of them. Pick a theme and build around the theme. A horde Skitarii army will not benefit from Ryza or melee-orientated or vehicle-orientated (self-repairing or cognis weapons) Forge World Dogmas, for example. And you do not necessarily need to build hordes of 20-man Skitarii squads just to use those Enriched Rounds or Galvanic Volley Fire Stratagems, not if you just want to play with Robots, gunships/bombers and tanks, and the vehicle-orientated Forge World stuff will benefit them more. If you're going Ironstrider-heavy, you might want to add an Artisan Tech-priest and make a gunline where he buffs the Strength of cognis lascannons. Though if you stay back, you won't get the re-rolls unless your opponent comes to you, so it depends on what you want...and let's be honest, there's little reason to buff the Strength of lascannons because they'll still be wounding your vehicle targets on 3+, so the only reason you'll do that is if you're firing 6 twin cognis lascannon shots at Toughness 5 Death Guard Terminators or Adeptus Custodes or Gravis armored Marines, which is...pretty specific - though, there's a good argument for buffing twin cognis autocannons to wound Toughness 8 vehicles on a 4+ instead of 5+, and Toughness 4 Marine Equivalents on 2+. There is also the option of taking a Logi Tech-priest to make a block of Ironstriders or maniple of Kastelan Robots even tougher. You can even do both if you want, but remember that your Artisan Tech-priest will be inactive for an entire turn if you switch to the Advanced Part, so yeah, the stacking of buffs will probably only happen from the second turn onward, though the Logi Tech-priest will probably improve the survivablity of that squadron. However, you can only buff ONE Core unit, so no cheating your opponent here, and just put a marker or colored dice to indicate the unit that's being buffed (preferably the same marker and colored dice you use to indicate the Holy Order of your Tech-priest and whether he's in the Initial or Advanced part). My advice is to keep it simple, though, so you'll probably just want one Holy Order rather than attempt to stack as many buffs on a single unit as possible, because it'll often backfire. Your opponent will usually be smart enough to target the super-unit with his biggest guns like melta, plasma and lascannons, in which case your Logi Initial Part isn't going to help you much. Maybe they might even snipe your Tech-priest, in which case I hope your whole plan didn't revolve around him buffing your dudes. Hell, you could even not bother with the Holy Order and character buffs and auras, so that your Ironstrider army will be more mobile and can move freely without having to wait for the Tech-priest to buff them. Or perhaps you want melee, in which case you can forego the whole blocks of infantry and make use of multiple small units in the form of Electro-priests, Sicarian Assassins, Serberys Calvary, Sydonian Dragoons and Kastelan Robots. I mean, you can take large units of Electro-priests if you want, but keep in mind they will be susceptible to morale, and you'll want to make as many charge rolls as possible to ensure at least a few of them reach the enemy. You can also focus on a melee Skitarii list (Sicarians, Serberys, Sydonian Dragoons, Pteraxii), and in this case you'll want to focus on the +1 WS Doctrina Imperative and not bother with the shooting as much (if possible, keep a Skitarii Marshal near your Serberys or Sicarian Infiltrators, or at least a Skorpius Dunerider near the Serberys or Sicarian Infiltrators to give them the Data-tether keyword so that they can still shoot properly, but that won't be a problem for your Sicarian Ruststalkers or Sydonian Dragoons because they don't have ranged weapons). Holy Orders might be helpful, particularly the Genetor one, but not very necessary as your Tech-priest isn't likely to keep up with your melee units rushing up the table and charging anyway. And if you put him in a transport, he can't use his aura abilities, so...yeah.
And now that you can choose your own Canticles and Doctrina Imperatives, it should be easier to keep track of them - you'll obviously want to select the most relevant ones for your turns. For example, if you're going second, you'll probably go for Shroudpsalm and Bulwark Imperative in the first battle round, and then switch to whatever Canticles and Doctrina Imperative you see fit as the battle progresses. Oh, you'll probably be in melee in the third turn, let's buff their Strength and WS. Something like that. And keep in mind the separation I outlined for you earlier - Servitors, Priests (including Datasmiths) and Robots are Cult Mechanicus. Everything else is Skitarii. Simple.
I hope this helps!