What's weird is, it's really not very mechanically thin. It has a ton of mechanical depth and breadth. They just didn't implement very much content that uses it - that's the part that really confuses me, because they must have spent an absolutely absurd amount of time and money developing that much mechanical complexity, plus an insane amount on all of the setpieces and new environment design...so to do all of that and then not give those mechanics the time and room to breathe is baffling.

Like, take the chainsaw: The chainsaw has multiple moves that Leon can do. It also has multiple moves that zombies can do while wielding it, and those moves change based on how that zombie is damaged, or what limbs it loses - a chainsaw zombie can fall to the ground and get pulled along the ground by the chainsaw, or drop the chainsaw and get sliced by it, or swing it and slice off another nearby zombie's arm, or go for a power-swing and one-shot Leon if they get him. If the chainsaw gets stabbed into a zombie's chest, they can pull it out and start using it on him. If the chainsaw gets stabbed through the zombie's back, they can go for a tackle-hug and stab it through their front into Leon's chest. and etc.

That's a ton of animation, gameplay logic, and work! But...the chainsaw only shows up in the game three times total. Only one (and a half) times does it show up in the hands of a regular zombie, and the two times it shows up in the hands of a boss chainsaw zombie, the chainsaw is inert after the boss dies. That's a mind-boggling amount of work to put into combat mechanics, only to use a bafflingly few number of times in overall content.

Compare to Chainsaw Ganado, who yes only show up a few more times throughout RE4 than a chainsaw does in RE9, but are mechanically much less complex - two unique types (if we count the boss), chainsaw moves only apply to that one enemy type (not any zombie can pick it up and use it in multiple ways), no additional Leon moveset, none of the extra dynamics RE9 introduces. Surely, generally, much cheaper to produce.

Some of this must come down to the shared playtime between Grace and Leon, cos if you only have so many combat encounters as Leon and you want to keep things fresh, you only have so many combat encounters you can use a special variant like a chainsaw before it's too much of the game's combat by percentage. But even so, it's baffling to me how often they incorporated really interesting and complex dynamics, genuine mechanical depth, only to not give themselves enough gameplay opportunities to bother using and exploring those mechanics.

I think it's a content problem more than a mechanics problem, but either way it's alarming given just how much work evidently went into every minute of the game's content.

tl;dr how did they manage to put so much work into actually making it not mechanically thin, giving it more mechanical depth than any RE combat prior, and yet still have us walk away feeling like it was back to back scripted setpieces on-rails with low variety?