I have flown with people from all over the world and I have also noticed that most of the bloody awful ones have been trained in the States. However, some of the very best I've flown with were also trained in the States.
What does his tell me? Not a lot really, but everyone who has learnt abroad has had to have had some "recurrency" training of some description. Whether it's explaining the differences in R/T or airspace.
But the big one is teaching them how to navigate visually, rather than beacon hop. That is one that has come up each and every time, but only with people trained in the US.