> Nix does not guarantee any Nix derivation/package will seamlessly run on Linux, Mac, and Windows. So in this one aspect it is less capable than Docker, i.m.h.o.
Nix runs on Windows exactly like Docker runs on Windows— only inside a Linux VM. If you ship a Linux VM on Mac or Windows like people usually do for Docker, you're free to run the Linux version of a Nix package on those platforms.
> i.e. if a package depends on the systemd package https://search.nixos.org/packages?channel=unstable&show=syst... , Nix will not automatically find a replacement to run the package on Mac. But it may be possible to manually work around this with https://github.com/LnL7/nix-darwin
Nix-Darwin doesn't do anything for packaging issues, it just offers an alternative module system for declaratively managing services and configurations in a NixOS-like way.
Docker containers also don't normally do any kind of service management, they're single-process images. They're not really comparable to orchestrated services managed by NixOS or Nix-Darwin modules. But if your concern is just shipping the same thing and you don't care about what's managing the services, a whole NixOS VM isn't any less efficient than some other VM in which you run `docker-compose` or Kubernetes.
That said, there are Nix-y ways to ship one or more supervised processes in a way that's portable across the platforms that Nix supports. One way is with a manually tailored supervisord config with a nix-shell or something like devenv.sh. Another strategy would be to leverage something an abstraction layer like this one: https://github.com/svanderburg/nix-processmgmt