In addition to what the other replies mentioned, it's also possible to fall back to other methods like flatpak or a container (which programs like distrobox make very easy to set up and integrate with the host) in case a package in nixpkgs is either non-existant, outdated or broken and you don't have the knowledge or the time to create / modify the derivation yourself.

For instance, personally I have everything gaming related set up inside an Arch container so I can trivially follow upstream (or the master branch for Mesa) without any friction and it works wonderfully.