When I first came to Pathfinder Second Edition, I saw the Inventor and though "wow, this sounds cool!" and then saw that there was an armor innovation and though "wow! even cooler!".
Little did I know I had saddled myself with what is the worst class in the game AND a character fantasy that is extremely hard to replicate with any other option.
What someone thinks when they hear "armor inventor" will, naturally, vary from person to person. For me around five years ago, it makes me think of characters like Iron Man, Techmarines from Warhammer 40k, and general concepts like durability and being an absolute wall. Nowadays I'm just disappointed.
The Armor Innovation is inherently held back from that fantasy because, for some reason, Inventor is geared towards damage regardless of innovation. Further, while 8hp isn't the worst, it isn't great either, and having some of the worst save progression in the game, being a MAD class (INT, STR/DEX, CON, WIS) all combines to make Inventor's chassis really bad for not even tanking, just wanting to be durable in general. The only feature that can really help out is Shield Block, but I have a lot of fantasy issues with it that I'll delve into later.
Alright, so the big armor fantasy is really clashing with Inventor's chassis and features, but maybe the Armor Innovation's got enough strong features for this to be workable?
You start with heavy armor AC at least, but I swear, armor's modifications are one of the worst pools of options I've seen in this game.
Initial Modification, your options are one of: a grouping of two elemental resistances, an athletics bonus that becomes equivalent to Furious Bully but is worse than Kineticist's Skill Junctions, a stealth bonus of the same nature, or a +5-foot speed bonus that is +10 in overdrive.
Those elemental resistances aren't doing anything unless you know that you are fighting enemies with those damage types*, or you just take them and hope you get lucky (this is a bit of a theme with Inventor). You can make a grappler build but then you're praying that 1 AC compared to most classes is enough to compensate for your atrocious defensive scores, and there's only one feat, Oil Fire, that specifically interacts with grappling. Since Oil Fire is Oil Fire you pretty much don't get any grappling feats from Inventor.
Breakthrough Modification, after struggling through six levels of not really contributing anything, maybe this will be the game changer? Options are: Antimagic Plating, a solid stealth improvement, a physical resistance modification for each type, increasing the elemental resistance to be your level, the Bulwark and Entrench trait, and a better speed boost.
Antimagic Plating is honestly not bad but it comes too late to save Inventor's terrible saves. Camouflage Pigmentation is fine, I guess? 7th level is when we finally get some physical damage resistance! For one damage type! And there's no flat bonuses like with the elemental ones, even in Overdrive! Is resistance to a single physical damage type really that strong? Champion is getting Expert in armor at that level and Armor Inventor is getting bludgeoning resistance? The elemental resistance increase is alright if you, again, know what you're fighting*.
And somehow, that's not even the most infuriating breakthrough modification to me. That title goes to Heavy Construction. So Inventors with the Armor Innovation are supposed to be good at inventing armor, right? So why then, does it take until 7th level to get the Bulwark trait, which is on a 2nd level item, full plate. Sure you get Entrench, but is someone seriously going to tell me that Entrench takes what is a 2nd level trait all the way up to 7th? This really screws over Power Armor Inventors, Inventors are already incredibly MAD so not being able to dump DEX is really painful. Oh yeah, that's another thing Heavy Construction doesn't do, it doesn't transfer the 1 DEX cap into a 1 increase in the AC bonus, so you're never able to even THINK of getting rid of that 1 DEX investment, so much for being an ARMOR INVENTOR. You have to use an Armored Skirt to do it for you.
Now to explain why there's a '*' after every mention of 'know what you're fighting'. It's retraining! Specifically, Reconfigure, which is Inventor's retraining feature at 3rd. Now while I could go into how this is an abysmal feature in the modern context of Animist existing, I'm instead going to explain how I believe this screws over the armor innovation at 1st and 2nd.
For the elemental resistance initial modifications, not having Reconfigure at those earlier levels means that, unless the DM/GM tells you what you're fighting, or there's some story reason, you are locked into those modifications UNLESS you get a week to retrain. Modifications that, again, have a good chance of contributing nothing to your performance. Now once you get to 3rd level, you then get to deal with spending 1 day just for the opportunity to make a HIGH DC Crafting check to retrain ONE modification, but that's a big rant for another time.
The last group of modifications, Revolutionary, is still somehow infuriating to me.
Automated Impediments - Equivalent to a 2nd Level Guardian Stance Feat.
Energy Barrier - Using up two modifications for decent elemental resistance is way too little, way too late.
Perfect Fortification - Actually reasonably-leveled equivalent is 2nd Level Guardian Stance Feat.
Physical Protections - This one, this one in particularly is just actually baffling. Guardian gets this at 1st Level. What is a 7th and 15th level class feature for Inventor is at 1st for Guardians. What an utter joke.
While I originally wanted to detail the Armor Innovation's feats, this essay is long enough as is so I'll end it by saying: in my experience with Inventor in general, and specifically with the Armor Innovation, I have learnt that this class is a joke. That is how Paizo has treated it with a Remaster that fixed none of its core issues and failed to bring it up to the same power level as the other classes, and the release of classes like Animist, Exemplar and Runesmith that have eaten heavily into Inventor's niches of retraining (Animist specifically), AOEs on a martial and item modification. This definitely reads extremely harsh but I don't hate Paizo with a passion because one of their classes is a joke, I hate the fact it took me so long to realize that Inventor was a joke.