I don't think guix does system configuration at all. And I don't mean "what software needs to be installed" configuration. I mean, configuration system that says "this is a staging environment, this app logs at DEBUG level via syslog, syslog gets configured to ship errors/exceptions to XXX, app uses local cache not cluster, credentials get pulled and decrypted from ABC".
This kind of configuration goes one level above package management, whether it's guix, apt, yum, pacman, or anything else.
Edit: you actually said benefits of guix in the original question - it gives you the way to make a localised installation of something you want. Chef/Puppet cannot be it on their own.
What Guix and Nix offer over puppet et al is a greater reliability in reproducing an environment - by eliminating the side effects that could lead to errors, or possible configurations and package combinations which have not been tested.
The fun part is that configurations can be treated exactly like packages and vice-versa. A "package" can depend on a specific configuration too. After you realize that package management and configuration management are not really disjoint ideas, it's the only sane way to think about reproducing software reliably.
This kind of configuration goes one level above package management, whether it's guix, apt, yum, pacman, or anything else.
Edit: you actually said benefits of guix in the original question - it gives you the way to make a localised installation of something you want. Chef/Puppet cannot be it on their own.