Among contributors (and maybe even among the whole community), unstable is probably more popular on NixOS than the stable releases, so if anything it has more eyeballs.
The distinction also matters less on NixOS anyway, since you can (generally) freely mix and match packages from different releases.
Lots of Nix packages are totally unmodified, but many also have to be patched in able to cope with the read-only package store.
On the other hand, NixOS is, like Arch, a distro without any defaults aside from systemd. So Nixpkgs doesn't include patches that try to reshape upstream packages for the sake of some particular vision of the ideal desktop experience or whatever.
The distinction also matters less on NixOS anyway, since you can (generally) freely mix and match packages from different releases.
Lots of Nix packages are totally unmodified, but many also have to be patched in able to cope with the read-only package store.
On the other hand, NixOS is, like Arch, a distro without any defaults aside from systemd. So Nixpkgs doesn't include patches that try to reshape upstream packages for the sake of some particular vision of the ideal desktop experience or whatever.