The Bell-Hiller mixer is a must go upgrade for any intermediate/advanced flyer. With this mixer installed, the cyclic response is more responsive, and you feel the Ufly will performs more "follow your fingers".
This mixer is a mechanical system which increases the cyclic response and reduces the amount of rotor rotation needed before swash plate movement is transferred to the fly bar. Besides, there is no change in stability with the simple addition of a Bell-Hiller system. Fly-bar is still doing its job in stabilizing the helicopter.
In short words, this decreases cyclic response time giving the pilot quicker and more precise control but not affecting the stability.