They do very different things even if the outcome is the same. You are not rollingback your system by downgrading each package. You are statefully changing your filesystem. Rollbacks in Nix and Guix are internet free, atomic and reproducible because they amount to changing the target of a single symlink
It is quite basic still and historically had some problems with reboots, but lately it has gained a lot of attention and bug fixes. I have no experience of runnit, compared to systemd is leaner but in my experience there’s no big feature lacking