RE: Tower Pro Mg996
The centering issues are caused by a heavy core in the motor causing an overrun when the sticks are moved fast (Such as in a recovery from an "Oops" moment).
There are a couple of other problems with SOME of the manufacturing of the 2 models mentioned.
1 - The Potentiometer is of a very low quality and wil wear out fast - this is the thing that determines the positioning of the servo output shaft. When the "Pot" fails, the servo can just continue to rotate until something gives (usually the plane as it hits the ground).
2 - the Wiring is poorly supported at the circuit board and can break free very easily. This is a fairly simple fix by opening up the Servo and using some acid free silicone to "glue" the wires up for proper support.
I've used plenty of them - both 995's and 996's but nowadays, I prefer to spend a little more for long term reliability.