I’ve been using Steam in FlatPak on NixOS for a couple years now.
The only games I’ve found that didn’t work were due to anti-cheat rootkit stuff, which would probably be a bigger deal if I cared about online gaming. And I’ve had to change the Proton version a couple times, because the beta (default) seems to break a game occasionally. Overall: it’s astoundingly good compared to where it was 5+ years ago.