So you know 3 sets of patterns?
Why do you say the Chang Hon Tuls are the hardest to learn?
I don't know them all well enough to perform at a poomse competition, but I'm working on it.
I would say the Chang Hon forms are hardest to learn for several reasons. Start with the fact that there are more of them (24, vs 8 Palgwe or Taegeuk, plus the 9 BB forms) and then consider their length; up to 68 steps, vs the 20-30 in the Palgwe and Taegeuk poomse. And I personally have always found the combinations to be more difficult to execute properly, especially some of the slow, balance moves seen in the Chang Hon tul.
I think that the Chang Hon forms are more "complete" in that there are a wider range of techniques used than in either of the other sets. The Palgwe forms, for example, don't include something as basic to TKD as a roundhouse kick.