That is very true. There are many NHL players that would suck in the KHL, for example.
and vice versa.
I don't think you'll get may arguments that the NHL is the best hockey league in the world by a fair margin. Leaving out style of play differences and that some players are better in some leagues, the NHL is the most difficult to make, the toughest to play in and have success - by far.
I completely agree with the social factors impacting one's ability to play well in a new country/culture. Playing at an elite level is hard enough on-ice, but being able to do that with the distractions/adjustments off the ice is different for everyone, impossible to quantify.
Jorgen Jonsson is a great example. There were some NHL games where he looked dominant, best player on the ice. Other times he looked disinterested and lost.
On the OP, it's a very odd question since it's implied that a career had to have been shortened due to injury, illness, playing elsewhere. Obviously we exclude players like Grabner and Skinner from the list so we are forced to look at European players who played SOME NHL. A much harder answer with little certainty one way or the other.
I think Valery Kharlamov was the best non-NHL player I ever saw, technically he played less than 100 NHL games - as BurrowsIsBeast points out. Certainly Mikhailov and Petrov as well, from the same time period.
Based on the success Makarov, Fetisov, Larionov and almost-Krutov had in the NHL after their peak playing years, we can safely assume that those Russian players would have been incredible in the NHL as well.
But now I'm way off topic.