Game crashed on launch using regular proton. With Proton Experimental bleeding-edge branch the game works without issues. Be aware that graphic quality settings are not available from the main menu, you have to start the game and then hit Escape key.

Note there are some black boxes in the game that may be mistaken for a graphic error (but it is by design and looks the same on Windows). There are a couple of spots in the game where the shadows flicker, but that seems to be a bug with the game and not Proton.