I actually think that's a hard progression -but it's not the type of hard that I struggle with -as that progression is an exercise in pure coordination, which can broken down into it's constituent parts and slowed down and practiced until you get it the muscle memory and coordination and then slowly brought up to speed. but it is still very hard and an accomplishment to play it of course.
The riffs I think are insanely difficult for me are the type of riff using shear agility at speed that don't necessarily translate to a slower speed correctly due to string tension/bounce back, speed bending and feel etc -there are some riffs, you have to learn with a measure of speed to get the feel correct to even sound like them. -those type drive me f'in crazy