I ran into several issues while having package = pkgs.nixUnstable; in my nix configuration.
One of them was warning: unknown setting 'extra-sandbox-paths'. But also segmentation faults while rebuilding my nixos and build errors with some of the hardened.nix options too.
Therefore I decided to switch back to the 20.09. defaults: nix.package = pkgs.nix;. Then all of the problems disappeared.
I guess some of the unstable packages are literally unstable at present. 