I agree that the Brick-tank is a problem unit, but today we had quite a confusion, when the tank was in combat with a cavalry unit with a hero on dragon joined.
The fact is - not only steam tank doesn't cause terror, I didn't find any mention of it being immune to terror! So the wimpy tank was terrified and made 1 less attack.
Maybe I don't know something? I searched the rulebook through and through, still found no mention of it. It's kinda counterintuitive