I have used NixOS for a while. But for casual desktop GNU/Linux users, it seem like solving a problem that doesn't exist. Have been using Debian on all my computers since 1998. Unless you are using Debian unstable and updating it daily, I hardly hit any breakage.
Servers and deployments is another story.
I use it to set up my window manager, fonts, Vim/VS Code/Firefox plugins, configure all my tools, set up user-specific systemd jobs, ...
I recently got a new laptop and I had my perfect setup in 20 minutes (install NixOS, git clone config repo, add new machine specific config (disks), apply NixOS config).
I appreciate that it's not for everyone , but it is brilliant.
In Debian we do: apt-cache search <search string>
It gives quick results.
The equivalent in nix is (correct me if I am wrong, it has been a while since I did this): nix-env -qaP <search string>
This command takes so long to execute. On a tiny VPS, when I did this, it ran out of memory. I wish things like this are improved.
The major one: package ecosystem. Nix has the largest and most up to date package repo of any distro [1]).
Nix is already somewhat niche, but the Guix community is tiny.
They also have a hard stance against proprietary software, so many things that are packaged for Nix aren't for Guix.
Second: it's pretty slow. A lot of the Nix functionality is implemented in C++ . Guix is all written in Scheme, and uses a rather slow implementation.
The main advantage is more coherent tooling and documentation. (Guix is much younger and doesn't have decades of legacy cruft)
Would love to see benchmarks of this if you have them.