I spent about an hour on this after your comment. Honestly, I was pretty lost in the filesystem for half of that time. I have never used either distro.

My copy of NixOS had no password on the root user by default, which is not ideal but I assume most deployments aren't like that (right?). I was able to become other users on GuixSD using the SUID's the distro ships with, but not root. Not yet. The surface is much larger on both of those distros than the mainstream OS's. I may be able to pull of a root LPE, but I'd need to look for a full day at least.