If you search RCU, you'll find some people who say Karbonite is brittle. I and may others have used them for years without any problems. Perhaps they are less strong than metal gear servos but certainly better than nylon. Generally the issues with breakage were in crashes or some other type impact to the gear train. I've seen some of the Karbonite discussions go just like some of the Futaba/Hitec/JR discussions. Most people, though, are happy with their Hitec servos.
As usual, Alan gave you a lot of good info. I didn't mention the vibration issue as your plane was neither a Heli nor Gas.