Why is Volkswagen considered unreliable by so many people in the US?

[lxl]

I am originally from Europe. I currently live in the US and I'm looking for a used car to buy. In Europe, VW cars are considered one of the best and are very popular. But here, most VW reviews are bad and they say it is unreliable. People report a lot of problems about their VW cars too, like failing engines, other parts etc. My question is why is there such a big difference between VW cars in Europe and US (Say, a 2000 Passat). I understand spare parts are more expensive since it is an import, but what is the reason for the seemingly big difference in reliability and the big difference in the perception of the VW brand between Europe and US?