Earlier quoted context omitted.
Unfortunately, Nixpkg doesn't check buildability on Darwin when merging PRs, so things can break quite often on the nixpkgs-unstable channel (compared to nixpkgs-unstable on Linux). It's not hard to rollback, though, but it does makes me never want to ever run `nix-channel --update`.
nixpkgs CI does check for buildability on Darwin, which can be confirmed by looking at any of their PRs. However, packages do tend to be get marked as broken on Darwin if it doesn’t build and the packager doesn’t own a Mac. I feel things have stabilized significantly in recent days, though. I haven’t had a single Darwin breakage on the unstable channel since the release of 20.09. Perhaps the Zero Hydra Failures effor…
Recently I had nomad breakage on -unstable caused by nvml support, which pulls in nvidia_x11, which pulls in linux_5_4, which of course not installable on darwin[1]. The maintainer fixed it very quickly (thank you!) but I still need to wait few more days for it to be included in -unstable.