I know it technically isn't true and I use the word 'practically' deliberately. The problem is that as soon as you opt into one part of nixpkgs (say a compiler) you end up implicitly opting into large parts of it because of common code in nixpkgs that make assumptions about undocumented magic environment variables etc that other packages (e.g. libc) may make use of or set.

I feel like a core problem though is the assumption you use Nix for everything - as soon as you try and pull in prebuilt pieces for pragmatic reasons, or output something you can use somewhere outside of nix, it starts making life hard.