Well, to be clear Health Insurance and the US companies that supply it often do so poorly (Kaiser isn't too bad except for mental health), but the healthcare itself can be top notch. People travel from all over the world to get the best treatment, but basic healthcare (preventative etc.) is expensive and not even uniformly acceptable.
The quality of health care is so high in Europe that I really cannot imagine any European going to the US for treatment apart from perhaps very few experimental treatments that are not permitted in Europe.
Unsure what it's like in Europe, but when my Canadian cousin had a brain tumor, he went to the U.S. for treatment. Canada's health care system is on average better than the U.S, but when you have a brain tumor, you don't want an average doctor, you want the best. And by and large, these are still in the U.S. because they can make an order of magnitude more money than they could in a country with socialized medicine.
The US has a much larger number of inhabitants than Canada does, so a much larger number of people that could be outliers in any one profession. On top of that the bigger pay draws the top players from Canada as well. So it is no surprise that that 'best' doctor is in the US, it would be far more surprising if they were in Canada (and stayed there).
Overall it's good in Europe. But there are some specialists and specialist clinics in USA (high priced of course!) which can be better than in Europe. E.g. Houston medical center is a big area of specialist clinics (e.g. cancer) and I can imagine rich people do medical tourism to this places. It's somehow comparing Europe Universities with US ones... they are not on the same level.
