Ok, I'm a newbie player in Descent and have played a few games already. However, I'm thoroughly astounded at how powerful the Overlord is. I've only played as heroes, and my sister as the overlord, and I seem to be flatenned to hero squash in the very first room in a couple of quests. The closest I ever got was in Quest 3: Grotto of Wisdom, when I almost rang all the bells and slayed Kisst-Haa. Unfortunately, the overlord spawned monsters in a corner, out of our line of sight, and crushed our weakest hero. Are heroes always threatened to the brink of death, and when they win, it's usually a very close victory? While the overlord can destroy heroes, with a couple of spawn cards and well-placed units, in the first few rounds? It doesn't seem right. 
Well the question is, is there a tremendous unbalance in this game, as the overlord outmans and outmaneuvers the heroes? Or is it meant to be that way, the heroes struggling to get through the dungeon?

Do different quests vary in difficulty (as I remember the quest I was flattened was in Quest 5: Last Wishes)? I remember the very first room in that quest was bristling with foul monsters. Also weak-armor heroes did contribute to my destruction.