They have a GUI package manager? That's news to me. When I tried it out last year, package management was handled by nix-env or configuration.nix.

Anyway, I found it much harder than Arch because it requires learning a completely new paradigm of system management. There is a very high upfront investment in learning multiple new concepts in order to change even the most basic aspects of your system - What are flakes? What are channels? What is home-manager?

Also, the documentation and online support is far more sparse. If you encounter an issue, it's much more likely that you can find the solution for Arch through the wiki or a forum than it is with NixOS. There also tends to be no single way to resolve a problem, and so a lot of the information you find online may appear contradictory and confusing.