Employees at two BMW dealerships in my area told me to stay away from the N63 and its variants, and said that many of their customers have had a bad experience with it. If they say that at the dealership, you know it's really bad. This was AFTER I bought a 550i, of course...
Also, read:
https://www.autoevolution.com/news/b...udy-97440.html
The Audi V8TT in the S6 seems solid - not many reports of issues on that one.