All deisels now run the CDI (or similar named) system to meet Euro 4 Standards.
The high pressures that theses sytems run at will cause problems on all makes, some makes it happens earlier.
Mercedes are notorious.
All vehicles will have problems of some nature, there are alot of parts to a vehicle, some are going to fail at some time or other, doesn't make it any more easy to accept when it happens though.
Luck plays a major part in ownership of any vehicle, but you can cut the odds down by going for a Japanese vehicle, as for VW and Merc, I have had many problems both with cars and vans from these manufacturers (inclusive of Audi and Skoda)