Pretty sure there are not hard facts out there about the reliability of the different brands,but generally,the reliability of Audi/Porsche is worst than the BMW.
Yes,the most problems of the BMW diesels are due to the implementing of the new "Clean diesel" system,or injecting the DEF,or Adblue fluid.Hence the multitude of problems in '09 and '10 models.'11 and up are pretty much free of flaws.
VW has the system working for a few years here,but the Porsche is just starting with that,so I wouldnt expect very clear skies.