I do the same thing. Being immutable is supposed to be great for updates. New image version and if there's a problem you can boot back to the last version no problem.

But functionally, like you I find Ubuntu server fine. I run apt update and upgrade a couple times a year and its local only with tailscale access.

I find these immutable OS's really nice on laptop or desktop. The home directory is the only thing that can be written to so the OS is supposed to be more stable and can't break easily