nixpkgs.config is not subject to the normal merging lib.mkMerge semantic.

{
  imports = [
    { nixpkgs.config.allowUnfreePredicate = x: false; }
    { nixpkgs.config.allowUnfreePredicate = x: true; }
  ];
}

The snippet above would evaluate to nixpkgs.config.allowUnfreePredicate = false instead of saying the option has been defined multiple times.

Furthermore I would like to see this option being merged with lib.types.functionTo lib.types.boolByOr, so we merge these definitions point-wise returning true on any predicate satisfied.