I would have to admit, I'd agree with the poll results: Planar Sphere is by far the most difficult, although certainly doable at lower levels. To me it's more difficult because I keep getting LOST! LOL
I think it's probably easier for magic-users (it seems each stronghold quest is geared towards the role, which is a VERY good game design). I play a berserkr, so I got DeArnise, but that was a "doddle" to me, even with the Umbre Hulk situation and the final "boss". Ranger SH is easy, if you ignore the Shadow Dragon (of course there's MAJOR XP there), and I have not yet had occasion to try the Paladin quest, etc.
Curiosity: what makes the Paladin quest so tough, since it ranks second on the "difficulty" list?