I didn't say WHM is harder in all aspects, I said it's harder to reach high DPS numbers due to the fact that they have more off-gcd DPS skills, their combined DoT potency is lower and every single DPS move can miss. ASTs have similar potency with only two DoTs and a burst spell, and the DoTs can't miss. For the healing part, WHM is easier in my opinion.
There is no secret formula to play AST, it just requires more insight on the other jobs in order to use the cards efficiently. Browse through the forum and you'll see repeated topics about basic card strategies and lots of tips are completely misleading (e.g. spear being useless, bole being too situational etc.). Every encounter changes how cards are optimally used, so AST is a very demanding job and most people are not willing to put the required effort. Not to mention the fact that some of th complaints are like this "I can't heal The Aery" and so on.