Just took a risk and updated my 3,1 MacPro running raid0 on an Apricorn card. First I installed the newest LittleSnitch update from today and rebooted once. Then began the 10.13.6 update. It got about 3/4 finished then displayed "the operation couldn't be completed. (PKInstallErrorDomain error 112.)" with a restart button. Clicking this led to a black screen and after about 5 minutes I decided to shutdown using off button. When I turned it back on, it began rebooting normally but instead of displaying the desktop, the installer started back at about the same point as it left off with the failure, saying 16 minutes remaining... and this time the update worked.
Spotlight went nuts rescanning everything (having lost my previous limited settings.) After I killed the MDS process and reset the Spotlight settings I like, the system is running fine now and the update apparently took although it does not show up in my updates history. I checked the console for new errors and it looks to be about the same amount of activity as 10.13.5. System reports it is at 10.13.6 and I have used the new iTunes 12.8.0 Airplay feature without issue.
The only real difference I noticed is that iTunes launches faster. System seems to be running great now--I've got multiple browser windows open, streaming my home security camera in one, running iMessages and Spotify, little snitch, a VPN, etc, and system and user loads are only about 5%-7% according to Activity Monitor, which is a bit less than what I'm used to seeing! Going to stress it tonight with Logic and see if it's truly better off. So far, I'm happy.