That's true but also that's means that you can tweak your environment with nix, for instance if you develop in python, while on Ubuntu/Fedora you have to go by hand/pip etc. It's IMO a plus, even if it's "hard to say they are independent pkgs".
I can also add few Gnome Shell extensions, Emacs packages and Vim plugins etc. They may "count less" but when you build your system, for instance via homeManager they count.
Having switched off Ubuntu when Canonical decide to leave desktop (because yes abandon Unity7 means leave desktop, and adding 10+year support to latest LTS means the same) I found a far more up to date system with a bigger package selection than Ubuntu, plus all the advantage of a functional OS (replication, never-brake updates, major distro version included etc)...
I can't really measure accurately nor package coverage nor popularity but I found NixOS all but certainly not an obscure distro... For GuixSD I'm a bit less happy since while I like scheme vs nix, I like Emacs integration and Emacs related packages it really suffer from having too few devs. But not NixOS. NixOS IMO is production-ready and a good choice other RH/Ubuntu for server usage...