[catern]

rpm and dpkg don't support having multiple versions and implementations of things simultaneously installed. Not sure what you mean by richer dependencies.

>You can also do unprivileged installation into a separate root with, say, fakechroot or PRoot

Yes, but that's the only way to (safely) use such package managers without privileges; so that's not viable to do for production.

>and you need something like that to install and run Nix unprivileged, don't you?

Install, yes. Run, no. Setting up Nix does require root, but once it's installed on a system it can be safely used without privileges by all the users on the system. That's very valuable in itself, but it's especially interesting when you consider providing access to a single Nix store over a network filesystem, or providing it to multiple different virtual machines or containers.

[gnufx]

On RHEL 6 I count eight different packages implementing the MPI standard -- multiple versions of three implementations of essentially the same API -- which can be installed together. The richness is that dependencies are not generally simply on packages; MPI packages provide things like "mpi", "openmpi(x86-64) = 1.10.3-3.el7", "libmpi.so.12", and config(openmpi) = 1.10.3-3.el7".

My Guix installation runs using a with privileged access to the store, and Nix is the same as far as I know. I don't know why Nix would be a particular advantage on our HPC networked filesystems, and it's not clear it's tenable for users to be able to DoS the store by installing an arbitrary amount of software in limited shared space.

It may well be that Nix makes the right set of trades-off against other possibilities for a given situation, but thta seems at least not completely clear cut.