In my opinion very hard dungeons must stay aside from main story quests. It meens, that main story quests and dungeons must be not very hard, but very hard dungeons must be just like an addition. They must not be compulsory for compleating. Hard for you? Dont go in this dungeons, play your game. You want to gain good gear and check your skills?- go to this dungeons. I think it is a good point