Yeah, Sovereigns and heroes in general are horrifically balanced. Very powerful early game, pathetically weak late game. Same is true of damage magic.
Someone used the word "mercurial" to describe the balance in this game, and its accurate of almost every aspect of the game: economy, troops, magic, heroes, summons, spell research, questing bonuses, etc. They all range from pathetic to ridiculously overpowered depending on how far into the game it is, because no one bothered to make sure they all scale at the same rate.
I've only had trouble defending my towns in one game, and that's because I got a real cruddy starting spot with 1 food and gold and no resources to expand to.