Ideally, every mass would be opposed by a counterweight placed exactly opposite: 180° away. This is not possible since the pistons occupy that space.
Any compensating mass NOT correctly placed (alongside, diagonal, etc.) has the effect of forming a bending plane with its axis a line through both centers of mass - a diagonal angled away from the crank's axis of rotation. The greater the misalignment, higher RPM, and greater mass the stronger the bending force. Bad.