The way I see it, Moghedien has such a weak will that she is bound to fail against skilled and stronger opponents in T'A'R. But she uses T'A'R for information, and is not really the type of fighter to be concerned with attacking an opponent directly.

Egwene did a pretty good job of running through the reasons why Mesaana could be beaten there, but it's interesting to note that on the flip side, Moghedien is a more difficult opponent because of the sheer fact that she would rather launch an attack (which in T'A'R can often be lethal) and escape rather than be goaded into a prolonged fight.

I never got the sense that Mesaana was more powerful in the Dreamworld than Lanfear or Moghedien - just that she could be goaded into a losing battle because of her personality.