They can only pull nine g's because they are designed to be the lightest possible weight that can pull nine g's. They are built thin but rigid, sort of like soda cans. Obviously tougher than that, but they're trivial to poke holes through. And those engines are delicate. I don't know if one piece of shrapnel would do the trick, but a small amount of damage to a fan blade can cause an imbalance that will rip itself apart at high rpm.