Kickstart is just RedHat equivalent of unattended install. NixOS goes way beyond that it also takes over CMS (ansible, saltstack, etc). You can for example modify one of packages (use different configuration option, or maybe sorry patch) and if binary version is not available in the cache it will compile new one. With home manager, it can also take care of your home directory, all configurations so called dot files, down to what extensions you want to have installed on Firefox.

I didn't know about this, thanks. But can you use it to change an existing system?