The main reason I don’t use them is because when I move my nixos config to a new machine as far as I know you cant get them to auto install. I have to remember which ones I had installed and redo them manually.
Which is why if for some odd reason I don’t want to just install from the nix pkgs repo. I use app images. I can keep them in a directory which I can just copy over to the new machine with my nixos config files.