I think there's some bug with regards to weapon handling. At least, once I had equipped a double handed weapon on a hero, I COULD NOT get them to equip a shield and a single handed weapon. They would wield the shield alone, or their default starting weapon, that's it. Equipping a single handed weapon, or another 2 hander, resulted in the icon getting lit but the character not equipping it.
If I'm having that much trouble, lord only knows what the AI is dealing with.