Sure! As a disclaimer, I'm one of the developers of HashDist. There's a longer explanation here: http://hashdist.readthedocs.org/en/stable/faq.html but the crux is that Nix enforces purity, whereas HashDist simply defaults to pure.

As an example, if you'd like to build a Nix system, you're going to need to compile or download an independent libc. HashDist is capable of bootstrapping an independent libc as well, but we default to using the system libc.

We choose to seat our "pure" system on top of the system compilers and libc, but by default, install our own Python and other high-level libraries. We also can integrate with other package managers, and there's an example branch in our repository right now showing integration with Homebrew.