For large enterprise use, Nix is going to need to mature to a point where it's possible for enterprises to host a Nixpkgs cache locally, using a binary repository manager like Artifactory or Nexus, both for sheer cost (to save on bandwidth of thousands of servers downloading updates from the Internet) as well as risk reduction (in case the public binary caches are unavailable at critical moments). If this is doable today, it's not really documented.
I'm sure the project will get there... eventually.