Which heroes do you have in mind?
Because in some cases, that is intended.
You see, quest bosses need to be beatable by all classes in order to progress. So while challenging, they should always be manageable at that point in the game if you use the resources at your disposal.
And most regular heroes themselves are also mostly unchanged, just generally stronger or endowed with a new ablility or two.
There are, however, some well rewarded side quests (Beast of Legend) or infrequent bonus encounters (Hildisvini) which are meant to be hard, and an achievement if you beat them early.
But you are free to go around them until you are ready - kind of like the Hydra in TQ.