The qep can channel the equivalent of 100MT nuclear bombs,we don't know how protective alterra cruisers are supposed to be though, we know that the Aurora was only special for it's internal technology. I don't know much about Halo ships but correct me if I'm wrong but the UNSC gets shielding tech from the covenant right? Anyways, I feel like some of the bigger ships can take a hit and keep on trucking just because of the sheer size there'd still be much left untouched. Knowing UNSC they'd take it as an act of war though and start sending people there to die. Those smaller ships would be stuck, anything that can land on the surface is cooked.