I think that it's a great project, and I hope it succeeds. My sense is that there is more momentum around Nix, so for a lot of uses it just makes more sense.
Guix and Nix both have the same issue imo, which is using a loosely typed language with an odd syntax. I feel like something both strongly typed and with a more common syntax would be easier to edit and faster to evaluate.
So, I actually learned about Guix via GNU Shepherd. It sounds like NixOS just uses systemd, which I don't love. Not in a dramatic way, and I'm currently running systemd, but it does break the Unix philosophy.
A Haskell-based package manager would be pretty dope (seeing as that's the gold standard for that sort of language). I wonder if someone's working on it.