The packager always should "explicitly require" what are the dependencies in a Nix package... it's not like it's a choice, if there are missing dependencies then that'd be a bug.
If the package is not declaring its dependencies properly then it might not run properly in NixOS, since there are no "system libraries" in that OS other than the ones that were installed from Nix packages.
And one of its advantages over AppImages is that instead of bundling everything together causing redundancies and inefficient use of resources, you actually have shared libraries with Nix (not the system ones, but Nix dependencies). If you have multiple AppImages that bundle the same libraries you can end up having the exact same version of the library installed multiple times (or loaded in memory, when running). Appimages do not scale, you would be wasting a lot of resources if you were to make heavy use of them, whereas with Nix you can run an entire OS built with Nix packages.