I'm a complete beginner but, for example, comparing Nix's Firefox and Guix's IceCat you can see that with Nix you have to fallback to shell scripting for some tasks while with Guile you can perform those operations with Scheme.
No they aren't. I was pointing to the fact that in Guile you don't need shell scripting where you can use functions like 'mkdir', 'symlink', 'copy-file', 'file-system-tree', etc..., so you can use scheme with all the support Emacs, Geiser and guix.el give you.