Eh, I find that monsters break relatively easily as well. They just do a lot more damage compared to undead before they break and run.
Yeah, that can make them tough to completely wipe out. It's not uncommon to need three or four battles to completely wipe out a big squad, especially if you don't have any cavalry for the finishing punch. And if all you have is a few small units, you can take a LOT of damage before you get them all.
