Is there really anything to be gained by placing the propeller at the top of the vertical stabiliser?
To place a heavy mass, part of which is even rotating, on a long, flexible beam is generally not a good idea. The engine mass will reduce the eigenfrequency of the vertical tail, inviting early flutter, and any imbalance in the propeller will provide the initial excitation which flutter needs to develop. Also, placing heavy masses away from the center of gravity increases the moments of inertia and makes the handling of the aircraft needlessly sluggish. The designers must feel some pain to pick this unattractive engine position.

Reasons for this unusual engine placement could be:


Better aerodynamic control for ground handling. With the control surfaces squat in the propeller's slipstream, ground handling of seaplanes on water is much improved.
Better visibility for the pilot: Instead of a center engine placement in the nose, a tail location gives much improved visibility.
Less FOD (foreign object damage): The raised prop position makes it less susceptible to being damaged by debris kicked up from the ground.


The first reason explains why most designs with tail-mounted engines are seaplanes. The engine location on the Trislander, the three-engined version of the Britten-Norman Islander, was picked for the last two reasons and because the designers felt that flutter was not much of an issue with that compact, slow utility aircraft.