"Necropolis" - changes to creatures of levels 6-7.

But the fact that a banshee is made tanky with such an aggressive ability is strange, to say the least. HP and defense should be below average.

Why is that? The banshee is not a dd in any way; the strength of the ability is designed precisely so that it does not depend on the unit's stats. Thus, one can charge into battle with them; nothing forbids it. Their damage comes from са.
And for Heralds, it's the opposite — own spell power is zero, therefore, they should have decent damage.