Since Ubuntu 15. one of the changes Ubuntu made is the switch from “upstart” to “systemctl” in order to start/restart/stop services. So in order to restart your Unity Desktop session, the correct way to do it:
jorge@buntop:~$ sudo systemctl restart lightdm.service
So far I have not found a keyboard shortcut for restarting Unity. In the past the keyboard shortcut for restarting gnome desktop was Ctrl+Alt+BackSpace. That no longer works. However, I’m sure you could assign your own keyboard shortcuts in “Keyboard” application -> Shortcuts.