I FLY _INDIGO
B707 was the last aircrft which did not have powered ailerons and elevators. It used servo tabs to lesson the force required. After that all aircrafts have used hydraulic powered flight controls. The amount of power that is needed and reliability required may be the factor favouring hydraulics. B787 is using electrics in place of pneumatics created so many problems with batteries getting burnt. If flight controls were powered by electrics it could have been fatal. But in future with more advances in technology anything is possible.