Had problems installing it too -- got stuck in a reboot "Unable to Install" loop. Once I broke out of that, it installed fine the second time, although I made sure to do each update separately then (i.e. Safari update, iTunes Device update, then Security Update).
I will add, the Snapshot feature of APFS really helped on this one, and I was impressed with it. When I got into the loop, I rebooted into Recovery Mode, selected "Restore from Time Machine Backup", selected the internal source drive, then selected to restore from the Snapshot which the system seems to have made automatically prior to the update. It restored the drive in a matter of seconds, I rebooted, and did the install again. Very slick.