[chassis]

DPF failure
EGR failure
turbo failure
DEF tank failure
DEF pump failure
NOX sensor failure

The items listed above failed on my Touareg, each of them is part of the diesel emissions control system. Read the VW and Mercedes forums and the same things fail with high occurrence on DEF-using vehicles. The high rate of failure of these items is why the reliability is questionable. Actually, there is no question, these vehicles are unreliable.

edit: the turbo is not strictly a part of the emissions control system. It is a high failure frequency item on the VW 3.0 TDI.