The trade off here is about the fact that the stipes needs to be well founded into the ground. When nailing the wrist and then hang and nail the feet on a standing stipes, the stability of the stipes should not be a concern. In case the cross is laying flat, it is more difficult to lift it with the condemned on it, since a substantial part of the lenghth of the stipes must be slid into the soil. But on a flat cross, nailing could be easier, because of the stability that is offered by the firm ground.