Every few minutes the mouse input would stutter and would need a few seconds to catch back up. Additionally, the game doesn't grab the cursor properly, as (most of the time) when I look far to the left it stops at a certain point and I have to hit escape and move the cursor back to the middle of the screen to look further. Same with the other directions.

Not sure if it's related to proton or not but after playing for about 5 hours straight the frame rate dropped to way below 60 fps. Before that point it was consistently above 60 fps.

The game crashes when selecting quit from the main menu, freezing the screen and requiring a reboot (I haven't tried killing it with a process manager because it gets stuck behind the game's window, so I can't see it and select the right process), and I've experienced a few (2-3) crashes of similar nature while playing.