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Yeah, with comparable armies, a mage hero can't compete against a warrior. It reminded me of the Necropolis campaign in Tribes of the East. The last mission, where you have to defeat a demon in a castle. Even the Banshee's scream, which reduces morale by -6, didn't work against this parasite. I had a 32nd-level Necromancer with a scream of terror and high morale, and I still had to buy units from four different life castles and subdue his strongest unit to win the battle.
In general, it's better not to make assumptions, otherwise they'll do something like they did in Heroes of Might and Magic IV, where the hero on the battlefield can use the "drug addict's path," eat a bunch of potions before the battle, and then single-handedly control a single Barbarian who can take down two dozen black dragons in one turn.
Also, it's a bit annoying that the hero becomes weak without an army. Heroes of Might and Magic IV was better in this regard, but on the other hand, a high-level Necromancer like this one, with summoning magic and a Phoenix, can practically solo defeat all the neutral creatures on the map, as long as you remember to cast a shield on the Phoenix and take one unit by subduing it.