vibration
Big vibration problem. Also on these types there is a loss of energy imparted to the rotating slipstream, curable to some degree by contra rotation which complicates hub and gearbox adding further to weight. All turboprops and turbofans have an issue that the vector sum of aircraft speed and tip speed will be near sonic or else low rotational velocities and large areas have to be resorted to. What a drag. Added to which Richard Shevell has passed on and William G Practice (Mr propeller part-time consultant "whizzkid") is 87. And even Bill is not a Fan of Fans.