The author has had much better luck than I've had, apparently. Tried it twice (last night and tonight) on openSUSE Tumbleweed + AMD graphics, first time with the latest stable Proton, and second with Proton Experimental. Performance on "high" settings is still somewhat worse than I'd expect, and the game freezes for me whenever I'd go through anything resulting in a loading spinner. Oh, and enemies are invisible, but apparently that's an issue for a lot of folks on Windows, too.

On the bright side, with Proton Experimental, multiplayer features do work, which is awesome. Hopefully more EAC-using games enable Proton support.