Security Update 2018-003 10.13.6-Total O/S failure requiring reinstall

I initiated the installation of the latest security update for High Sierra. It all went normally. I then went back to the App Store Updates section to check it and there was no indication that I had completed the installation. The update/install option was still there as if I hadn't installed it at all. I thought that was odd, so I rebooted the Mac. I got a grey screen with a huge number of crash report log entries. There was a dialogue box option giving me a restart/shut down option. I shut down the Mac and rebooted with the same result: the O/S wouldn't boot. I spoke with a senior Apple tech. rep. He told me to reboot using Recovery mode (Command+R). I got the desktop showing, but everything was extremely slow. He did a screen share to observe how the software was acting. He concluded that there was a corrupt file and I'd have to do a complete reinstall. I had a bootable backup and three hours later, I had copied that to my Mac and successfully cloned it. (using Super Duper) No boot/performance issues now, but I'd like to know if anyone else had an issue with the install of this Security Update. It's now there in my Update section and obviously I'm hesitant to try it again. Any insights would be appreciated. It's the first time in 20 years as a Mac user that I've had such a complete software failure.