Actually Nylon geared servos do BETTER under vibration because they "give" a bit without damage.

Conversely metal gear servos wear more under vibration.

However even a grossly out of balance system is not likely to cause problems with either, as long as the engine is not in direct contact with a servo rod/horn, etc...

If you are talking about say a throttle servo, things may be a bit different, and you may want to make sure that the throttle servo can "give" a bit to deal with vibration.