It's definitely the hardest one I've taken there. Most of the difficulty comes from the density of the lectures. She moves fast and takes it for granted that you're piecing everything together as you go. You're probably not, but at least you can go back and watch it again if necessary!
Hinton's Neural Network class was very challenging for me too, mostly because many of the concepts were unfamiliar to me. But again, I could re-watch whatever I needed to in order to get it.