My first thought when reading this is that the URE specs don't necessarily have to be realistic. From what I've seen they haven't changed for ages. That just sounds strange. It's an interesting topic, but it would be better if we had some hard facts regarding URE's rather than product specs.

FWIW, the HGST SAS drives are another order of magnitude better, 10^16 This gives a 2.5% chance of URE rebuild FAILURE if these were available as 6TB drives, and one out of six failed in a RAID6 array.