Well, for me it's not the posting that's the problem. I ride western and english both, and I post during both becuase my horse has such a horrible trot. (Straight shouldered fellow that he is, not his fault!)
For me it is the constant contact required in English. Your arms have to literally be like a rubber band, and a super sensitive one at that. Unless we are working on collection and rounding in western, I ride with a totally loose rein and for me that is waaay easier.