Home-manager has a lot of options for configuring packages available through it though. You have to manually write config files in your global configuration.nix for “what color should be the fg in this terminal”, while home-manager will provide an attribute even for that.

Also, you can use home-manager from within your configuration.nix, that’s how I use it! Then you don’t have to deal with it separately and everything is at one place.

Yeah, I admit that home-manager is already decked out with all of these options. There's some more manual effort that goes into using configuration.nix here. That being said, I've been seeing more software being packaged up as nixos modules. For example, you can configure a git installation globally now.

I'm a single user and still use home-manager. And I wouldn't be surprised that's the majority of cases.

The thing is that while NixOS configuration.nix also contains modules for software, it primarily concentrates on global services and might not have modules for programs that are meant to be run under user.