This game used to run flawlessly a few months ago but now it will crash on startup more often than not. It will start, show the Cryengine logo, then crash. I've restarted the game multiple times and it will occasionally start fine and run without any problems. As far as I know, any problems you have after the initial startup is with the game itself (it's playable but very buggy) and not proton.

Side note: on windows, you can press keys while sprinting or freelooking but for some reason you can't with proton. This is a minor inconvenience when you're trying to see the names of who's around you.