I'm the author of that PR. I apologize for the forced usage of btrfs. This was one of my first delves into the nix language, and I literally copied the digital ocean backend and changed the API calls (and the d.o had a hardcoded filesystem and I wanted btrfs and didn't know how at the time to move that into the configuration level). After the 17.09 release of NixOS is completed, I'll put some focus back in that PR and hopefully get it merged upstream.