RE: Spektrum DS821 Sevos Junk or good
The servos are good -
the intended application is as noted for setups -really with std length arms and definately not heavy surfaces
The inertia on a hard thmp down will possibly shear them
Why?
depending on the servo - you get white rather soft nylon or filled nylon (all colors) or metal.
Most earlier JR servos used plain nylon gears and the usual failure was the next to or next to next to last gear .
The bright white filled outputs and the black outputs are filled plastic and harder and ---more brittle--- Great servos but adding a 1.5 " arm and a ball joint on top of that is a recipe for disaster -if a shock load is introduced
If you fancy youself as a hard flying 3D type avoid plastic gear trains on any thing over -say 8-10 lbs-- any brands -