The problem will be related to obsolete applications that can't be properly authenticated by the boot loader.
Try booting up with Verbose mode turned on (Command-V). This will tell you where the process hangs.
In my case, it turned out to be the Eltima SyncMate app, but for good measure, I went through the apps and removed anything that was ancient, unnecessary, etc. I then went through Application Support, Launch Agents and Preferences folders in both /Library & ~/Library and removed anything that was no longer relevant (i.e., for all the apps I had removed).
It would be awesome if these tasks happened automatically when removing apps, but at least I know that I've done some housecleaning. If I had done that prior to using Migration Assistant, there would have been no issues. And the system now boots remarkably quickly (27" 5K Retina iMac, 32GB RAM & 3TB Fusion drive).