Actually looks kinda appealing? I'm using the Flatpak on NixOS right now, and it's left a bad taste in my mouth. Tracking upstream Mesa drivers and pre-configuring MangoHud makes a pretty strong case for using the Snap.
Personally I run Steam (and pretty much every other gaming related program) through an Arch container using distrobox[0] on my NixOS setup. It runs without any issues and integrates with my system very well.