Fix for Mojave Failure with Fusion Drive?

The internal fusion drive on my 2014 27" iMac was thoroughly corrupted by my attempt to install Mojave. The machine had been running High Sierra very nicely. Although I had two full SuperDuper clones of Macintosh HD on external drives I was unable to repair the drive and install Mojave or High Sierra.

I took the iMac and one of my clone drives to MacMedics in Severna Park, MD (maybe the best authorized Apple repair shop in the country!) They were able to repair the fusion drive, but they were unable to install Mojave; each of two attempts corrupted the drive. They finally repaired the drive a third time and re-installed High Sierra, and, thanks to my system clone, they returned my iMac to me with all data intact.

I don't know how common this Mojave and fusion-drive issue is, but MacMedics reported that they had seen other failures similar to mine. And an acquaintance of mine, a long-time Machead like me, has experienced the same failure with a 2015 iMac with fusion drive.

Mojave installed without incident on my 15" 2017 MacBook Pro.

I would be curious to hear if anyone knows how widespread the fusion-drive problem is, what causes it, and if there is a workaround.

TIA!