If you natively install one of each, Server 12 Core and Server 12 GUI (Full) you will find that Windows Update from each lists roughly the same number of Security Patches and correspondingly after installing each security patch Server 12 Core requires the same number of reboots as Server 12 GUI (Full).  How can this be if 80%-90% of the security patches for Server 12 do not apply to Server 12 Core?