He has a good point here. In the challenges, the items that flew had to do their job while flying. The brick sat (as bricks do) the blender blended, the toaster toasted. For a sledgehammer to work it has to swing.
Swinging a sledgehammer is a VERY different challenge than blending, toasting or bricking. That's a moving (heavy 2lb or 12lb) weight. Instead of simple mechanics and complex electronics, it would likely need simple electronics and complex mechanics.
Imagine if Dave had a sledge instead of a pen for the duel in the air with the airsoft tail-gun...