(Author here.) I'm biased but I do think that the blending between Guix and Shepherd is unique and supports a level of integration beyond what's usually achieved in distros.

The services-in-containers extension you mention is a good example of that: it is implemented on the distro side (in Guix) without any modification to the Shepherd itself. The good thing is that the distro is in a good position to determine how to set up that container since it knows about the package being run, its configuration, and so on.