That's my other issue. Nixpkgs just isn't as rich as AUR. I'm willing to use mostly nix OR mostly pacman, but not some 50/50 split of both, which is what the current situation tempts. I'd be extremely pleased if there were some sort of adapter plugin that let nix manage external packages so that I'm not forced to choose.
I think it's fantastic that we have a declarative, deterministic, idempotent package manager that works across all these OSs. I've used it happily for years now without ever bothering to try NixOS.