I’d say NixOS is great for servers, mostly. Only having to worry about certain things (secure boot with custom keys, FDE, partition layout, network, sshd, firejail, etc.) once, and then replicating the same setup on another machine is waaay more convenient than going “I wonder what I was thinking when setting up this machine” once in a while when looking at some machine again you haven’t touched in some time. When it comes to desktop usage, the whole thing does not feel as magical - configuring system options in e.g. KDE is still a lot of clicking around in a GUI. I still use it for my desktop machine, just so I don’t have to think about another distro.