[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.