A poll on AdvRider puts the rate at 7%. Somewhere they came up with BMW's estimate of 3% and admit more people are likely to respond if they do have a problem and therefore do not discount BMW's numbers. I know a couple of people who have experienced a final drive failure, but know many more that have not.
A tech at a large metro dealership said he sees about 1 premature drive failure per year across the whole BMW line. He said a leaky seal is more common but that rarely results in failure unless ignored.