Just played some games on Linux Steam yesterday. It works so smooth. The Steam client is actually buggy at times with a tiling WM, but all the games worked great.. even the oned with Proton.
It's actually amazing how well games work under Proton. Even a significant number of multiplayer titles just work, despite anti-cheat. There's some "random problems" sometimes with some titles, sure... but that's also true on Windows. Interestingly the problematic titles are more or less exactly opposite to Windows - new titles tend to work well on Windows and perhaps have issues on Linux (requiring some specific launch options or Proton version, perhaps graphical glitches). Meanwhile older titles are often problematic on Windows, but are less so on Linux. Might just be my selection bias though.