I've been experimenting with Nix on Mac OS X lately and it works fine. I've heard that it works on FreeBSD as well. The big gap is Windows.
The good news is that you can integrate your language-specific tools with Nix as well, such as has been done for Haskell, node.js and other things. (I'm looking at it so that we can integrate our Dylan stuff with it.)
I'd love to have the time and the resources to deal with more OSes. :) 20 years ago, I had to keep stuff running on Solaris and lots of other platforms. About 20 years ago, I still did some work on VMS on actual VAX hardware! It wasn't that long ago, that we had the possibility of BeOS either. Comparatively, we have quite a monoculture (of POSIX) these days with Windows being the non-POSIX representative.
Maybe unikernels like OpenMirage will help make things interesting.
And thanks! The work on Dylan is a lot of fun and keeps me semi-sane by keeping me busy.
The good news is that you can integrate your language-specific tools with Nix as well, such as has been done for Haskell, node.js and other things. (I'm looking at it so that we can integrate our Dylan stuff with it.)