I've had a similarly painful experience upgrading last week. Though it doesn't seem quite so bad as the posters above, and after making a few fixes most everything is back to normal.

My one remaining serious annoyance is that my external monitor color settings are screwed up and there appears to be no fix. Reds are purple and everything is just a little washed out, which is a shame for a 4k monitor that was beautiful with Mojave.

Strangely, right before the computer restarts, or if booted in safe mode the color starts to look perfect again, but I can't seem to replicate that in normal operation.