I've been lurking on the BMW 1600 site for a while and it's interesting to hear the folks there complain about their bikes too. Here's one post from the 22 page "List of Problems" thread (red text)...
...Kinda hilarious... if you did a global search & replace of BMW with Harley Davidson on that site, you'd be hard pressed tell our sites apart (except for ours is better and Harley's rule
)
In ‘75 I trained at Butler & Smith in Compton, CA. as a BMW mcy tech, and I serviced them for 6 years; although I don’t have any experience with their machinery built after 1980. As Mark has pointed out, these things are machines, and as such they will breakdown.
One of the more common issues from that era was the failure of the spline coupling between the ring gear and rear wheel; often at around 30k miles. The OEM fix was to replace the ring and pinion. There was an aftermarket vendor that would machine out the worn spline from the center of the ring gear and weld in a new replacement; this guy did a remarkable job reconditioning this piece, and as I remember it he was very reasonable.
There were other issues of course, but I was always amused whenever I had to take care of a drive coupling, because in that era the BMW folks were the authority on shaft driven motorcycles, and that assembly wasn’t particularly reliable.
The BMW Z8 would be an example of machines gone wild if this was about 4 wheels.