Guix configuration is in Lisp, a well-established language (60 years and still strong). Moreover, Guix package code is written in a clear, declarative style. Nix package definitions, on the other hand, typically embed shell scripts in them.
Yes, but systemd is missing and most drivers because they are not gnu.
The choice to use the Shepherd rather than systemd is motivated by a vision that we can get better integration using a single language and set of tools all the way down. As an example, this gave us services in containers several years ago:
https://guix.gnu.org/en/blog/2017/running-system-services-in...
Overall Guix System alone gives a coherent and unified view of the system; systemd also does that to some extent, but it's not in as good a position as Guix's declarative OS configuration to do that. For example, this is the kind of system exploration that Guix enables: