For my first few years of NixOS I didn't understand the point of the NixOS stable releases, since even on "nixos-unstable" I found that if my nix config evaluates, then it'll work. And in the very rare case things broke, I could easily rollback.
NixOS stable, for me, provides API stability. I can leave a machine auto-updating, and be confident that my nix config will continue to be compatible, and thus build.
Thanks to the release managers for the work that goes into this!