NHF – new heroes for Heroes V 3.1.

A deadly attack also has a negative aspect: when dealing normal damage, Death Knights can eliminate many more creatures...

And a grenade in the hands of a monkey won't be of much use :D - I mean that you need to know how to use the creatures' abilities correctly, and even considering the fact that there are situations where Death Knights can eliminate many more creatures with normal damage, the ability is still strong, and the described drawback is not that significant, I think.