Interesting comparison, because they're 2 very different works! I play both sonatas (and the Pastoral too) in their entirety and found the 2 movts. very difficult for very different reasons.
The Moonlight 3rd is difficult to execute well because of the obvious technical chalenges in the passages. And the level of musicianship required in Beethoven is very demanding, and this is no exception.
In the Schubert, you will need to come up with the right kind of sound to sustain a melodic top line at the appropriate slow tempo. It's a much more similar kind of challenge to the Pastoral sonata. The middle section is just manic hellish! It's an incredibly powerful music, this.
More than anything else, I say they are very difficult simply so that they are given the proper respect they deserve. It is in no way meant to discourage.
I think one of the bad things about exams is the way grades supposedly "categorise" the difficulty of each work. I find that often people consequently consider such works as movts. from Schumann's Waldszenen, Romances and even the Pastoral sonata "easy" because they're not found in the AMus or LMus list. I for one can't imagine the Pastoral 1st movt. being performed well by just about any supposed 8th grader. But maybe someone will prove me wrong.