I had the same problem. Installed update, hung at about 70%. After 4 hours decided to reboot, came to the login screen, and after entering credentials the busy mouse pointer stayed there forever. Booted to safe mode, reinstalled.
After several days I thought well, let's try again. >> Same result. This time I did have a TimeMachine backup, so I decided to restore from there. The system prompted that the chosen target disk would be formatted, and I clicked Ok. Result: not possible to erase disk. After which my complete disk was no longer visible (fusion disk). I ended up removing and rebuilding the virtual volume group entirely trough a command prompt, after which I did succeed in restoring my iMac to a workable state.
What I don't understand is why this update is still online. Yes, there are not that many people which posted this problem here, but then again, most probably won't go there. Definitely big minuses for reliability and service there Apple!