I've seen this on two machines now - once when performing a manual upgrade using the ISO and once via WSUS.
Both times, the installer gets to 73%, then stays there - indefinitely.
On the manual installer, I can just click Cancel and after a short while, the installer disappears and I'm back at the desktop. But when installing via WSUS, there is no cancel button. I wonder if it's safe to restart the computer, or will that completely bork Windows??
Is there a log file for the 1809 update that I can take a look at to figure out what's going wrong on these two PCs?