Last time I tried Nix I eventually got the Nix environment working, but ran into MacOS-specific issues trying to install relatively common development-related packages like Ruby and PostgreSQL.
It's waaaay better than it used to be. A couple of years ago there were a lot of impurities which would cause everything to break whenever macOS was updated. But these days it just uses libSystem from macOS and the rest is managed by nixpkgs.