3 Engine Ferry
With checking approval on three four engine heavy turboprop aircraft that had three engine ferry approval I can say that the most dangerous part was the training and renewal which in two of the aircraft was done without a simulator.
The Rolls Royce Dart was the worst as the throttle of the simulatored failed engine had to be constantly adjusted to simulate the feathered engine during the takeoff roll otherwise a negative torque situation would develop that would eventually lead to a loss of directional control. A real life three engine ferry was far safer.