Nix works well on mac, very similar to Nix/Linux for the most part. There are some missing packages, but the common ones tend to be fine. Its worth using the Determinate Systems installer to avoid reinstalling Nix on every macOS update though.

Nix-darwin is good, and I use it, but it is nowhere close to NixOS. I think there are some options I've set through it that macOS keeps overriding, so the declarative configuration drifts from the real one eventually

I think the only real issue with Nix on macOS is that Nix can eat through storage quite quickly, and storage upgrades are pretty expensive on Macs. This might push the balance back to an fanless ryzen build