Like I said, my BW unit has done 170,000 miles and is still going strong with no attention other than replacing the ATF and replacing the rear seal. the reason they can go wrong is because so many people don't change the oil on schedule and because so many will abuse them by testing the brakes on a single roller pair or driving with a failed half shaft or missing prop shaft. Over extreme technical terrain like rock crawling, having one wheel frequently in the air might eventually lead to trouble too, but for more normal off roading and on-road driving, it's a superior unit to the LT230.
The viscous unit has internal clutch packs that act like a LSD. You can turn a wheel by hand if I jack it up, but only slowly (about 2 rpm, perhaps) - the viscous unit is always engaged, and does not open or close the diff like a locker.
)and the Discovery wasn't seen as being in a class where the refinement and better road handling of the BW mattered. It was deemed reliable and beneficial enough to be used on the P38, which though an unreliable car at the best of times, had few transmission problems and suffered almost entirely to electrics and EAS faults.
