For me, it's the difference between taking your medicine a bit at a time on your own schedule or taking it all at once as an unwelcome surprise. Sure, setting up file system mounts or adding udev entries is easier to do once in Ubuntu than in NixOS, but I only need to do it the one time with NixOS. Thereafter, the config serves as both documentation and backup. For a hobby self hoster like me who occasionally shoots himself in the foot and has to rebuild a system, it is ideal. I don't know if it really saves me time, but I do know it saves my sanity.

I am no nix whiz, but it's the only OS I run outside of containers. Anything I can't easily get with my nix config I shove into a container, run it as a quadlet, and call it good.