Problems with Heroes of Might and Magic IV.

in the late game of Heroes 4, the hero becomes very weak. It should be designed so that he looks decent in terms of both HP and damage (like a commander in Haven). But this is just my opinion.

In my opinion, it's the opposite... While the hero is still small, he has little impact on the battle. But when he reaches level 15-20 and pumps up his combat skills (and other skills of his choice), he becomes a walking death machine... But combat skills are essential; without them, the hero is easily defeated.
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I remember even in the "The Gathering Storm" campaign, there was an archmage who was given five types of magic but no room for combat skills. They completely ruined that hero...