What do you mean? NixOS you basically can reboot in the latest working version, so the downtime is likely minutes at the worse. Also snapshots in Btrfs/ZFS are really fast to recover too, since they're basically a pointer (again, at worse you will have minutes of downtime).

I had updates in Windows take quite a long time to complete, even on SSDs, and in Windows you also can't update the system while running (it must be during reboot). And since you can't use the machine, this can be considered downtime too.