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.

No argument there, I definitely agree, however its easily my favorite package manager. And lets me keep nixos config similarly.