The quality of Nix packages for Darwin is quite bad compared to Linux though, I don't think there's a core maintainer that runs Nix on Darwin.

By quality do you mean quantity? Sure some packages are missing on Darwin, but those that aren't don't seem "worse" in any way.

Broken builds often take quite a while to be noticed, and there aren't many people to reproduce them. Recently, as an example, `ncdu` broke.

There is one: @reckenrode. He's really good. But yeah only one person who knows nixpkgs inside-out runs Darwin. Low bus factor.