No worries, UEFI is definitely a "milage may vary" kinda standard.
I've personally only used NixOS and not Nix on a different distro by itself so I'm less familiar with that setup. No system is perfect for all use cases, but that's sorta the point in Unix-land. I personally have been gripped by NixOS and having to go back to Fedora for some of my old servers has been a pain. They use it like a buzzword all the time, but declarative administration is so awesome. It does have a heck of a learning curve though.