Cam tensioners seem to go bad and the engine has to come all the way out of the car. Basically anything that goes wrong with the car, they have to take the engine out to fix it. It's not all that unreliable just incredibly difficult to work on.
I mean, these things cost about 40 large when the were new. Any car that is that expensive from the factory is going to be expensive to maintain. I had shopped a few before we bought the v6 passat, the W8 is incredibly alluring but in the end I got scared away from it.