Stealth kills would be a reasonable buff but I think it's more how you use them. Damage is low due to daggers being low damage. Armor is generally low due to build factors. However, stealth characters often have two or three jumps and often polymorph so can go invisible and fly. This means they can strike then jump out of danger. It's a different way of playing and invites even more damage to the other characters but is it truly weaker? Hard to say. Now, the final fight, I'm no fan of. From my experience, the only way to win it is with magic and mostly three-level source magic at that. In that battle, my stealth guy was mainly the bring them back from the grave guy and there is no place to jump to safety.
