I have collected over 2000 films in the last 20 years. I thought I would 'backup" my copied backups to mkv files. Due to the age of the disks, I have had about 10% failure rate. The failures I believe are caused by the BR dye breaking down over years of storage and ruining the disk. It seems Memorex and Ridata have been the worst accounting for over 95% of the failures. The best has been Verbatim LTH type dye with few failures.
More would be in the trash can if it wasn't for a little trick I came across. I found if you blow your hot breath on the disk in small area at a time, rotating the disk and then immediately rub the disk with a clean soft tee shirt, the errors go away on the next read. I'm talking about those pesky "error occourred while reading at offset XXXXX" It may take more than once to clear them up but it does and I don't know why. I have also found that putting the disk in second or third drives will sometimes allow a good read. Just MHO.