I posted this in the analysis thread for a simple post, but I also thought I'd post it here and get other people's thoughts. For reference, I spent most of my time with inventor focused on armor-inventor, mostly because I love the hell out of an iron-man style inventor concept.
So I know I'm super late to this discussion, but I felt a bit let down by the 18th level armor-inventor feat. Both weapon and construct got a spammable "ultimate" ability, whereas the armor got effectively one-shot big DR.
Now with unknown changes coming to the unstable trait, it's hard to say for sure how that ability will turn out, but that discrepancy felt odd.
I'm also very glad that unstable itself is seeing a change. Inventor is arguably one of my favorite classes in PF2 thus far, but my biggest gripe with it was how much of the design space revolved around various abilities/feats that all keyed off the same essentially once per encounter resource. There's a plethora of things you can do, but you can effectively only do one of them (2 with the 14th level feat) in a given encounter. I would've much preferred to have unstable work on a case-by-case basis (I.E. Each action with the "unstable" trait would only need to roll if itself had already been used previously), but I also recognize that that would add a lot of bookkeeping that PF2 has thus far been trying to avoid.
Maybe instead of a hard-and-fast one use, then you have an 80% chance of never using it again for the encounter, develop some kind of ramping difficulty? I.E. Your first use is guaranteed, and then maybe make the flat DC 5, then add some number to that for each use of further unstable actions. I'm not sure how to balance the "risk" element, as losing access to it entirely seems fair but it also means that a single unlucky roll early (like rolling a 1 on your first check) suddenly cuts you out of the unstable abilities entirely, but if you don't make the risk bad enough (like say oh you take this small amount of damage if you fail) then players will just muscle through and not care about the risk at all.
Personally I think my ideal version of this would be some version of what I described above, where you have an "Instability" counter. For each unstable action after the first, you add 1 to the Instability. At the end of the action, you roll a flat check, with DC of 5+2xInstability. If you fail the check, you take 1d6 damage for each point of instability. And maybe as part of the class itself you have a cap on how unstable your invention can become before it stops functioning, with that cap increasing as you level so you can push it further.
As a final note, I'm also not a fan of the Infinite Invention class feature at 19th level. I completely understand the thematic and design choice that led to it, but at the same time I can't help but feel that it almost removes the "identity" of each sub-class at 19th level. For the first 18 levels they all have a distinct path and flavor and then at 19th level they just unite into one homogenous blob. I'd almost prefer this as an 18th level feat instead of a core feature? If someone wants to take this path with their inventor by all means, but I feel like having it as a core capstone-style ability is weirdly out of flavor for the one-minded obsession that the class shows up to that point.
Just to be clear: I absolutely love this class and would play it even if printed as is, I just hope that some things can be made even better. Overall I think it's a great class with a lot of awesome ways to be built, but armor-inventor definitely feels like it got the short end of the stick, and I feel like designing a significant part of a class around a single once-per-encounter resource kind of flies in the face of PF2's apparent design philosophy of trying to provide characters with a large toolkit to mix and match on the fly.
Actually, I'm playing a construct companion inventor, 5th level, and paid the feat tax to try and keep my companion useful in melee, but it just simply can't keep up.
Compare it at +11 to hit; when a fighter can get +18, and other classes get +16. That's just not viable, period. With no ways to boost attack, like other classes get potency, the construct companion is just completely not on par. Too, way many misses.
Failing your unstable locks out other abilities (hazardous repair, searing restoration, etc) and too easy to crit fail, doing damage to self.
While the class has some flavor, that flavor is backed up by awful mechanics and a non-viable progression, which can even punish a player frequently, just for making choices that utilize the feats it chose.
It's clear to me this class sub-choice didn't get the same treatment as other core classes in terms of balance. It certainly doesn't look playtested. It's been my only major disappointment of PF2E, and hope it gets some severe improvements in a new errata.
My biggest pet peeve with Inventors is that they are singularly focused on a single invention, they cant even mass produce it. Maybe im unfairly comparing it to artificers in 5e, but I expected Inventors to be able to modify, add, or create that can be used by not only the inventors but by the group. Much like iron man making multiple iron man suits , or making a unique weapon or gear for all the other avengers to use. It just seems like unless you allow a very liberal crafting skill process, your Inventor character is a one trick pony.
I know balance is important for PF2e, so its hard for me to imagine a way to rebuild the inventor class without severe repercussions. As it is im just adding a caster archetype to the inventor and just roleplaying it just like the artificer in 5e. I just wish the core inventor class had more feats that allowed it to modify not only its primary innovation, but modify all gear the group uses. Maybe they will add a sourcebook in the future that can expand options for inventors without having to dip into a caster class.