HERE IS THE "switch" I FOUND TO WORK AROUND MY SHARE>BLU-RAY FAILURES
iMac 2017 i7
64gb RAM
8gb video RAM
1TB internal Apple FLASH SSD
Mojave 10.14.6
FCPx 10.4.10
My project Library had 13 Story Lines totaling about 4hrs
I Exported each Storyline to Apple ProRes 422 and created a new Library stringing them all into one long 4hr StoryLine
I added about 60 Chapter Markers to that
I FIGURED THAT WAS THE SIMPLEST CLEANEST WORKFLOW I COULD CREATE yet I experienced intermittent variety of Disc Create failures and Share failures
HERE IS The FIX SOLUTION that helped me complete my .img and write my DL BD disks
The ChapterMarkers (CM) were causing the problem in two ways:
- It seemed if I Modified my ChapterMarkers to contain any numbers or special characters caused the create disk blueray to fail -- even using "..." in my last CM caused a Disk Create failure -- at least when I deleted the ellipsis the process completed and created a usable .img
- It also seemed dragging the CM point thingy below (to set a thumbnail) also caused the failures -- at least I had better luck when I did not drag the point to specify a thumbnail for scenes/chapters

If you are having these types of blue ray sharing failures crashing unexpected quits ect -- I would look at these alleged bugs and use only letters in Chapter names and do not slide the point thingy to select specific thumbnails
BTW: THE SAME ISSUES REPEATED IN COMPRESSOR 4.4.8 on Mojave
However running FCPx in BIG SUR (with the latest FCPx) rolled right through Create Disc and the Write process EXCEPT the BD would not play on Sony players (original posted problem) and the menus were all screwed up on my LG 4K BD player
LASTLY:
my Chapter Marks (on my finished 4hr BD) went way out of sync from where I set them in FCPx -- I had to guess and set them up to 10 seconds early at the end (and every time in-between) it was a real guessing game where they would actually start playing
worst software I ever seen on a Mac (pro user since the Centris 650)