Back when I was looking to switch distributions a year ago and it came to the choice between NixOS and Guix System, the latter unfortunately lost due to lack of features I considered essential for me. These were availability of proprietary packages (notably Steam, though I guess this could be rectified with a flatpak version or something), and no support for secure boot, which was the prime reason to switch in the first place, as I wanted to enable passwordless FDE unlock on boot for my machines (at least for the desktop, this should be secure because of fTPM).

Secure Boot is a bit of a more involved process with Lanzaboote, it's not just another "enable = true;", but at least after initial setup it just keeps on working.

I recently spun up another server for various uses, one being backups using restic. According to https://packages.guix.gnu.org/search/?query=restic, it's at 0.9.6 in their repos. NixPKGs has 0.16.5. 0.9.6 turns 5 years old this year.

The other services (yes, they are sketchy, but all GPL) aren't even in Guix at all. Yes, that's a network effect, but if switching the distribution forces me write half of it myself (exaggerating here) it's not suited for my case. The Nix ecosystem has issues but at least it enables me to build the system I want. Guix unfortunately is just another GNU project that's more focused on ideals than practical reality, which, given GNU's nature, is completely understandable and justified. But probably also the main reason for why in the real world, Nix is dominant in its niche while Guix System is a footnote.