Come on, they're against neutrals. There are Hunters to counter them. What are the Mistresses for? This is level six. They don't appear right away, to put it mildly. And the upgrade isn't cheap. So, using them for clearing is, at the very least, foolish, as usually everything is already cleared by that time. I didn't like them. And with slightly enhanced stats... what can you get from them besides analyses? Shooting is much more useful, don't you think? And they shoot quite well. Although, in principle, they are mainly needed because of their spells.
In my opinion, it's a weak upgrade; the developers didn't have enough imagination. They could have added another spell. That would have been equivalent. But as it is... in my opinion, it's terrible. They still don't fight very well in melee. They are much worse than Rakshasa/Nazgûl/other paladins. A quick attack is rare. I defeated 15 Mistresses with 60 mercenaries, with approximately equal hero stats and without using magic on either side.