Note you can use any Nix package in Guix...

Ouch. Is this at a legal or community level?

Community.

There may even be community-based sources for non-free packages, but they are explicitly ignored by the Guix project.

Anybody can run guix publish however and self publish their own non-free substitutes if they wanted, plus there's all the existing non-free substitute servers around, and you can try your luck with guix import from nixpkgs. The community won't help anybody if they ask for help with non-free software though.

I haven't missed any non-free packages since switching to Guix surprisingly, I always seem to find a free alternative though my daily req are pretty basic.

> plus there's all the existing non-free substitute servers around

Where?

> The community won't help anybody if they ask for help with non-free software though.

This, specifically, is where Nix has the advantage.