

Audio issues and periodic freezes, but performs well outside that
Entering a menu during gameplay stops music. Music does not start again, even if you change levels. Some enemy sound effects may not play as well.
Fullscreen does not remove the taskbar. Setting game to native resolution winds up with either the top or bottom of the screen cut off if you are in fullscreen or windowed mode, respectively. Windowed mode does not stop the mouse cursor from triggering the dock overlay if it passes by it.
Freezing/stutter during combat, particularly when enemies changed animations
Some of the issues with adjusting the game's resolution and fullscreen/windowed more are present even in Windows. However, Proton seems to amplify the problems.

Works great, aside from sound bugs and performance hiccups
Can't hear gunshots, some other sounds are completely missing (particularly music, aside from menu music). Volume slider in menu does not work.
Game freezes for about a second at a time when some enemies are killed.