The type problem is starting to be solved by flakes; the equivalent to the type `operating-system` is the type `nixpkgs.lib.nixosSystem`.

To analyze a NixOS system configuration, I look for all modifications to the `services` attr (representing all the services of the system, e.g. `services.mpd.dir = "~/Music"`). If I want to know even more I can look for all the `.enable` keys to see if any hardware/program/etc options have been enabled, and the sources are easy to read so I can open the corresponding module file in Nixpkgs if I want to know what a particular option does. The default NixOS machine does literally nothing so I don't have to worry about that.

Guix does look really cool, and I would try it if it had more support for nonfree software (I depend on some nonfree software in my day-to-day.)