I tried NixPkgs for a while on a mac about a year ago, but it wasn't really functional (I submitted a half-dozen or so bug reports, several were fixed, but I could see there was a long way to go towards a usefully working system...)
I use NixOS as my main OS though and it works fine.
The good news it that the darwin stdenv is now "pure" in the functional sense. It bootstraps using /usr/bin/curl, and symlinks libSystem into the nix store, but otherwise doesn't rely on macOS at all. That makes it much more resilient to changes in macOS and Xcode.
I'd say it is very much usable and in fact used in production.
I see this as a thing of relative scale. Most people aren't going to run Nix, and in practice, using Nix requires a fair amount of work in practice (often including upstream work). If you're prepared to go through all that, you can probably use Guix in principle, too. (Barring some technicalities, of course, but in general I think this is still true.)