I'm in the exact same boat.
Firstly, the engine is just fundamentally mediocre at handling anything that resembles standard FPS gameplay. Off acceleration, input lag, annoying movement speeds, weird animations, etc. Gunplay just feels like an afterthought. I instinctively quick load when a firefights break out.
Secondly, enemies behave in a way that's incredibly unsuited to what you can do in a combat scenario. Fast-moving mimics, warping phantoms, projectile attacks that take 75% of your health... I mean, it makes zero sense that I would approach two mimics with more panic/apprehension than I would a phantom - and it's because I know I'd be fighting with imprecise controls and enemy movement that just seems unfair. A seriously limited arsenal doesn't help either.
Thirdly, everything else that makes for great shooting mechanics are average at best: recoil, feedback and sound are all middling and offer little satisfaction.
And finally, dealing with enemies through stealth becomes extremely rote and banal (albeit still decently enjoyable). The AI is exploitable and options are limited. Proceedings for me had devolved into sneaking up on enemies and shotgun-blasting them for huge bonus damage. It was satisfying, but it didn't feel great.
I did play the game for 10 or so hours, though, so it does some things excellently. Art style, world building and level design are all fantastic and immerse you thoroughly.
It's definitely a good game at the end of the day. I just wish the gameplay loop was a bit more polished/enjoyable.