I have recruited a hero with the berserk ability. As I didn't have the appropriate equipement yet to make him into a solid defender, I decided to use it first with a bow to protect him while waiting to buy of find a good armor.

I then realized that the berserk bonus worked for ranged attack (bug or working as intented ?), and as losing the control for a unit that just fire his bow each turn is not as bad a problem as for a melee fighter, I let him go berserk each batlle.

Then something changed: during a level up I gave him the earth 2 promotion, to get access to the spells, having no other earth mage.

Since that, each time that I go berserk, he first cast stone skin on another character, and only after that does he starts firing the bow.

I think that the chechk that make berserk units unable to cast spells might not works for the turn where the ability is activated.