Interesting, that no one of dozen deployment tools introduced deployment-wide modules (of those nixos system closure would be submodule; there is already an example of that in nixos-containers.nix).
That would not only reduce memory consumption of multi-machine evaluation, but also open a space for hyper-modules with options out of the scope of particular machine (and in some sense being like Helm of Nix world), allowing declarative configuration of
VPN (or Tinc) network
trust graphs (for RBAC or just for deployment keys for password-less SSH)
distrubuted software which run on mix of dedicated machines and ephemeral containers
I think this is a decision of the developer. To my knowledge, Nix community projects have shared ownership. A good resource of available Nix tools is Awesome Nix | awesome-nix.