My biggest gripe with Nix packaging is that the tooling varies so widely per ecosystem. Aside from that, though, I generally find packaging for Nixpkgs to be much, much easier than packaging for most distros.
What kind of packaging experience are you used to?
The area that IMO needs the most work is the packaging experience, including the DSL itself... and the "I just want to build this source without having to become a black belt functional packager first" experience.