The complex part of Guix comes from it being a inmutable distribution based on the ideas from NixOS

That’s not the most significant factor in what makes it hard/different. There are immutable distros that come with less complexity and are arguably more immutable than NixOS or guix.

What actually sets it apart and can make these harder to use is:

1. They’re declarative rather than imperative. You describe the desired end-result rather than providing (or manually executing) the steps to construct it.
2. There is not a single global dependency dependency state upon parts of which any given package depends. Dependencies are explicit, direct and encapsulated on a level that’s as fine as you’d like instead (down to the per-file level).