Protoss player gets units in place to defend the Mothership, hits World Cracker, ability begins spinning up and Mothership is locked in place - can't move. Begins blasting and gradually wipes everything out underneath it.
As it spins up it's obviously going to be attacked, and since it's slow this has the risk of being a pointless suicide mission for a big expensive unit. To make it worth the effort, you could give the Mothership another ability: Everything within a certain range (maybe the old range of its passive cloak) contributes to one collective shield shared by it and its surrounding units. That would enhance the surround and defend strategy of the World Cracker, and the general usefulness of this big slow unit.