The times I use metal arms is not due to concern about splines stripping but due to concrens about flexing. A heavy duty 4-40 clevis is very strong as long as the plane of rotation for both the control surface horn and the servo arm are in the same plane. If the servo is installed 90 degrees to the horns rotation then the axial displacement of the rod can be too big with 3D type control surface movements and it'll cause the clevis to want to open up. In that case you would want to go to a heavy duty ball link and that is the reason for switching to a metal arm since the ball link will cause a plastic arm to twist and even snap off with heavy sharp loads.