My rcexplorer style tricopter with all motors on top flew perfectly with the KK2 board, but the Arducopter tricopter firmware doesn't seem to take into account that the mass of the yaw motor+prop causes the tail to initially move in opposite direction that the board wants to correct to. IMHO there should be some (tuneable) deadband in the firmware for this.
With the top mounted motor, I kept getting oscillations at almost all flyable gain values.
To confirm that this was related to the mass of the motor, I mounted the motor inverted and then it flew perfectly without oscillations. However, this setup is far from ideal for sport flying, because the prop is now more vulnerable to hitting the ground, hand launching/catching is more dangerous, and it requires much taller landing gear.
Does anybody know if this can be tuned/fixed somehow? I'd much rather have a top mounted yaw motor.