You don’t have to reboot to adjust configuration or update. At least not with NixOS, don’t know much about other immutables.

Why not an immutable OS for servers? It seems like an even better choice for servers than for a desktop to me. It’s basically just like docker/containers but for your whole OS, configuration-wise.

I love my declarative NixOS servers/systems, and would never go back to imperative setups, it just seems archaic. The only exception I have (for now) is my pfSense router, as I don’t trust myself enough to setup a secure router.