The problem is that this isn't a feature that it supports, it's just that someone noticed the way you can combine the existing features leads to this possibility. It's the same with a programming language - until someone noticed that you could use javascript and web browsers to make a PDF viewer or an email client, maybe there was no documentation about how to do that.
I think it's not so much a problem of Nix/NixOS, but a consequence of the power of composability. For the next n years, people will keep discovering new things you can do. As the knowledge spreads, people will work out how to document the features and present them in ways that can be easily grasped. But the final success of NixOS will be when every distribution adopts more and more NixOS-like composability and NixOS is well understood and even boring, like the final success of Unix.