I really like Nix package management. I really hate Nix configuration management. I already know how to configure everything. I don't want to have to figure out whatever abstracted config options some random package maintainer decided on.
As Nix (and NixOS) becomes more widely used, this is one place where we'll have to find a way to let users do what they know how, and stop getting in the way.
I wouldn't be mad if opening the system preferences and setting my timezone also updated my timezone in Nix. I am mad that I can't change my timezone when I travel, because the GUI picker doesn't know that Nix has taken ownership, so only changes in Nix take effect.