The usage of "unstable" itself is from an era where even stable versions broke a lot. This is not the case anymore. And stable repos breaking more than unstable repos in some distros atleast proves it.
NixOS benefits from the fact that it has unit tests and that all packages are built in a consistent way across all packages.
Archlinux is a bit more stable then NixOS unstable, but not significantly so. But more importantly if you do somehow manage to update to a broken build, it's take less than a second to roll back to exactly where you were before hand.
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.