I use the Futaba 3003 or 3004 servos, nothing fancy.

Now sometimes the throttle return spring may be too stiff and that could stress the servo causing the driver transistors to fail. Some guys use a thin wire to pull the throttle and let the spring return it. So in that case you don't want the return spring to be too stiff. Using the stock short throttle arm can be a problem as there is little leverage against the spring which makes the servo work harder too. If you have the spring on the throttle with the pushrod that opens and closes the throttle, you can pop the spring end off and leave it on there as a spacer and see if that fixes the problem for you or not.

If the airframe isn't very rigid in relation to the engine, the excessive engine movement can cause a stiff pushrod to strain the servo causing it to fail in the same manner. You basically have the engine vibration beating up on the servo via the pushrod then. So using a flex cable or golden rod might do the trick in that case too.