Not to put down Nix, but some of the arguments about other systems have counterexamples. For instance, it's common in rpm or dpkg distributions to have multiple versions and implementations of things, and richer dependencies than simply on packages. You can also do unprivileged installation into a separate root with, say, fakechroot or PRoot, and you need something like that to install and run Nix unprivileged, don't you?

There are comparisons with alternatives in at least the propaganda for Spack and 0install, though the same criticism might apply to those -- I don't remember.