When I say ADC is the most mechanically challenging, it's a combinations of things. One, since it is the most gold reliant class your farming has to be on point, both in actually last hitting and wave management. Two, you are almost always the #1 target for the enemy team's CC and asssassins, therefore your positioning has to be perfect or you'll get blown up and contribute nothing to the fight. ADC's also have to be able to orbwalk and kite constantly, where mistiming a click ONCE can lead to death.
I do agree that jungle is also challenging but not mechanically for the most part (some like Lee Sin and Elise are mechanically challenging. most of the tanks are not). The hard part about jungle is they have the most complicated decisions to make through out the course of the game. Knowing when and where to gank, clear paths, whether you can take dragon/baron safely, deciding which camps to smite, helping ward, deciding whether you can invade the enemy jungler... the list goes on.
I've been maining ADC ever since i started playing this game 2 years ago, so it's the only role i understand really (besides support, which i learned how to play to make my ADC play better). Like I said, I play a little mid and top from time to time and sometimes i can do well enough to carry games from these roles purely from my mechanics, but my lack of knowledge specific to the lane hinders me since i don't play the lanes often.