I think if your geometrical symmetries are ok and you have the appropriate mechanical advantage it is very difficult to break plastic gears on a F3A model.

Here is the 3421SAs on my OTOP, never had a problem and this is the glow version.

On a 2m 3D/freestyle model I fly now I consider seriously transforming its metal gear aileron servos JR 8411 onto 8401 plastic, some times I fly pattern with it. Metal gears are very sloppy and servos never center properly, specially for JR they lose their high performance POTs potential due to slop.

rgrds,
Nikos