I've been playing games such as Spelunky 2, Hades and Divinity: Original Sin 2 on Arch Linux with Steam/Proton, and it works seamlessly without any issues. I have Windows still just in case for some games, but I haven't booted it for ages.
Jedi: Fallen order worked out of the box, no sound issue.
Mass Effect: Andromeda works flawlessly, perfect frame rate, surround sound worked perfectly. I just got one crash to desktop over the whole playthrough and I suspect it was due to something else (updated packages while running the game). Incidentally, the game is great, esp. compared to the negative review; I'm still shocked by the discrepancy.
Same for Witcher 3, GTA V, No Man's Sky ...
More recent games often still have issues. RDR2 is just getting playable, but it's getting fixed faster. There's also less and less annoyances: just a few months back alt-tabbing was very dodgy, now it's extremely robust. Also multi-monitor support seems to work perfectly nowadays. I had some issues with controllers but I haven't tried recently.
It’s surprising that AAA games on Linux work better than ever these days even without native support.
Exception: Bad Unity games. However these often do not work properly on windows either
Has this been fixed?