I use nixos, not GuixSD, but I can speak to "what would be the advantages and disadvantages", particularly since guix is a reimplementation of nix.
The benefits to me are immense, because I travel between around six machines on a daily or weekly basis. Prior to moving to NixOS, I'd have to manually keep those machines in sync in terms of software installs, etc. However, after switching to NixOS, I have a set of files I version control which will declaratively build the system the way I want it.
So, if I install a new package and I'd like it be the same across all machines, I simply configure it, commit to git, and then pull down the config on the other machines and do "nixos-rebuild switch". Everything becomes exactly the same.
It's saved me so much time in terms of managing my computing life. I really couldn't see going back to a regular distro after using one which is declarative.
The drawbacks? Well, for one, Nix and Guix have various tricks with symlinking, etc, to make it appear to applications as if it's a typical Linux filesystem, but it's not. E.g. there's no /lib, etc. It's a little weird getting used to.
And with nix, there's the nix language, which isn't that difficult to learn but it's not my favorite. If the underlying language needs to be functional, I'd prefer a lisp over nix. Also, with nixos, there are too many different commands required to learn when you get deeper into it, but work is underway to improve that story.
At least for me...the benefits far outweigh the risks. I don't think I can ever go back. I think there's a real opportunity for a commercially-backed declarative Linux os...it really is a game changer and would do well in the enterprise.