There are some massive, incredibly fiddly consequences of using NixOS to run Steam games via Proton. A lot of stuff works, a lot of other stuff is a royal pain the ass.

I moved to flatpak for Steam on NixOS and couldn't be happier, except now the latest proton uses flatpak under the hood and flatpak doesn't support nested containers.

I hear the flatpak-based Proton works much better (and that makes sense) but I still have some titles that are broken with it but work with older versions.