hardware - afaik only lenovo(some say asus is worth to try - but no official linux support, framework is sturdy but feels cheap) is well know for quality hardware - others are questionable.
unfortunately AMD AI Max 390/2/5+ nor Qualcomm Elite 2 Lenovos are not here.
it kind of 'just works' if someone already wrote the nix code to do what you want it to do and put it in nixpkgs and you manage to find it and figure out how to use it. but if that isn't the case, good luck. i once spent almost a week trying to get a program to build and run properly under nix that could probably be installed in around 20 seconds on a osx/windows machine.
The quality of the managed / packages software, however, is still a bit subpar compared to Debian and Redhat.
2. nix is just package manager and configurator of sh which forces to write idempotent sh code with explicit dependencies(just good practices right? check guidelines of any non nix solutions and you will find out that 90% of these rules are just nix). nixos, nix darwin, nix home manager final artifacts are just just idiomatic dotfiles, so nix does not exist.