Monster heroes and their lack of defense
Take Tuatha as an example. A Troll champion with +6 health regeneration per turn. In addition to being a defender (+1 hitpoint per level) he also has the Tough trait (+3 hitpoints per level). Tuatha has 0 defense, and his only ways to boost that are spells, shields and maybe some accessories.
As a melee/tank champion, Tuatha gets flattened in battle. His defense is extremely poor, and his hitpoints are nowhere near enough what is necessary to survive against even basic militia.
There are plenty of other heroes that have to give up significant capabilities for defense, trading them away for seemingly weak talents. While it makes sense from a purely aesthetic perspective, I really don't see it working out balance-wise. The lack of defense only serves to expose how poor AI is at choosing weak targets. Meanwhile, if I face them in battle, they get destroyed instantly as lack of defense = death.
Perhaps something simple as giving them +1 defense per level would help.