The Morso like any piece of equipment is only as good as the craftsperson is experienced in using it. It is not as simple as delivering one downward blow with the foot and bobs-your-uncle, the perfect corner cut.
The Morso has two very sharp cutting blades. The inner cutting surface is flat whereas the outer surface is curved. To get a cut in which both sides meet flush you need to make sure that both cutting blades make the cut.