I can see where you are going, but in reality it's way more complex than that. Levy, mercenary or professional is not the important thing, but whether the army has a close bond with their commander. I also doubt the berserk response would be triggered if the the army was already uncertain and under heavy pressure.
However, this is probably impossible, as morale penalties were hardcoded in the previous games and I am not aware this is different in M2:TW.
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