My first GS, a 2002 1150 needed a final drive rebuild at just over 100,000kms. I realised there was a problem when the drive began to leak oil. The service manager at Auto Classic in Perth, a BMW master tech, rebuilt it for me on a Saturday morning and I did the remove/replace so a relatively cheap repair. My next BMW, a new 2006 GSA, had the drive fail in under 10,000kms. Apparently a number of early 1200 GS's and GSA's experienced the same issue. This time around I found it by giving the rear wheel a bit of a shake and there was noticeable side to side movement and a slight knock. Drive was replaced under warranty.
My 2011 GSA goes to its new owner today and 56,000kms, no issues and hopefully the 2018 GSA Rallye which I now own will be equally as reliable. Only time will tell.
My experience with BMW dealerships has generally been positive as long as the bike has a decent OEM service history and hasn't been flogged within an inch of its life. Just my two bobs worth.