I've only had two in-flight servo failures in many years of flying.
One was a mini servo on an electric model fitted to an aileron, it failed jammed at about 1/2 travel,
flew it home on the other aileron servo.
The other was on a friend's ARF model I was training him on, failed free wheeling (like there was no power on).
It was on the elevator, all I could do was cut the throttle & watch it crash.
I ask the question in the context of dual servos driving a one piece elevator. It's possible to create a 'sliding bar' mechanism so that if one servo seizes the other will still give a degree of elevator control. However, if a servo fails but does not seize, then the amount of remaining control is dramatically reduced as the 'failed' servo cancels out much of the movement in the functioning servo i.e. there's nothing solid for the functioning servo to push against.
A relevant question for me, my next model will have a prominent, visible single pushrod & horn on a one piece elevator. With the two examples of failures I had a sliding bar system of some sort would save the model if the servo seizes, albeit reduced throw depending on where the bad servo seizes. If it free wheels I would be better linking the servos solidly together so the good servo could drive the elevator & (hopefully) the dead servo.
John.