The old Hydra upgrade might not even jump to neutral archers or sorcerers, but simply wait and regenerate while the hero attacks enemies with magic. When there are only a few enemies left, they will regenerate more than they will die.
Witches are also not bad. They may seem weak, but they have enough health. In the beginning of the 3rd mission of the original campaign, 2 witches without any losses can defeat 120 Furies.
Each unit can be useful in a specific situation.