I can actually see both points. I think there probably ARE some generalizations that could be made - because I do think there are certain mbti types that seem overall to be very bouncy and not much phases them - but I also think there's something to be said about ANY type overcoming their issues. Basically, I think ANY type CAN be happy...I just personally feel that some types have a much harder time finding that (I'd say INFx in my experience ;-), and yeah, several others too), or it waxes and wanes for some types more than others.
And each type is going to have its unique 'battles' I think....for example my reasons for unhappiness would be quite different from an ENTJ's reasons. Etc. Or an ISTP may have no idea why certain things trigger my unhappiness. And vice versa. That kind of thing. (just pulling those two examples out of a hat...it's the same across the board)