RE: DS821 Servos
Absolutely. Heli specific forums are full of threads about them shearing the output shafts. The amps die too under too much vibration.
We have a simple test for them before putting them in a helicopter. Put them in a bucket of water, if they float they are safe to use.
I've had nothing but excellent results with Spektrum and JR equipment but those particular servos belong in 40 size trainers.