We're comparing it to Snap and AppImage. Flatpak doesn't have enough meat on it's bones (or goodwill in it's tank) to compete with the other cross-distro distribution options, and certainly not with native packages. I'm frankly not surprised that Steam is constantly broken on NixOS, Valve doesn't maintain it (and won't due to how insanely different it's concept of package management is).
Reality check: since the Steam package is broken so often on NixOS, I ended up using the Flatpak on NixOS instead (which works like a charm).
It's a uniquely bad format, almost worth spinning up a Linux VM to try it for a laugh.
I am not sure what you are talking about. It's actually a really nice format that uses OSTree.