I'm principally against abstracting package management across platforms, because experience has taught me that the software management subsystem of the target platform is the best system for that platform. I see no point in this abstraction since I understand that the best packaging format is the native format, and I've no problem mastering the native format.
I couldn't care less about NixOS, as I'm a UNIX (IRIX, Solaris, illumos/SmartOS, HP-UX) guy and always will be. I run Solaris 10 and SmartOS on my infrastructure, before that I ran IRIX and HP-UX as well on it.