Why is the jaw of a dogfish underslung? what disadvantages does it have? ... ?

First off, the jaw itself, is capable of projecting an impressive 30% of the total head length, which is over 10% more than the jaw protrusion of a lemon shark. Also note the teeth of Squalus acanthias actually interlock to form an almost uniform cutting surface. Finally on either side of the jaw you have what is referred to as labial cartilage. These structures fold out as the dogfish projects its jaw, essentially causing the jaw to take on a tubular shape, which increases suction pressure as the shark opens its mouth. As the jaws close these cartilages snap back into place in folds along the side of the jaw, which is why spiny dogfish, smooth dogs, nurse sharks, and other sharks that make use of this mechanism have such distinctive "cheeks." The disadvantage it has is that it cannot open its mouth too much.