The dynamics, which are not just the rigid body dynamics plus the aerodynamics of the vehicle but also include many structural modes of the rotorhead and blades, are not scale invariant. An RC helicopter is proportionally stiffer in certain degrees of freedom by an order of magnitude (in rotor angular velocity and thrust:weight ratio e.g.), and so can ignore dynamical factors that dominate the control and structural sizing for manned helicopters.

Do you think a PID control would would fail to be able to accommodate those dynamic factors?