As a shielder I have no problem fighting against another shielder. (Other than the fight being boring as hell)
So, I can't believe that a 2-hander or a polearm user would have any trouble against a shielder. My polearm guy eats shielders for breakfast.
Again, the only change I think is needed is to have shield speed mean something. If the lighter, less durable shields were faster, it would give people a reason to use them.