There are other such systems as you describe using 6DOF (like Alexmos, used in many DIY projects and commercial handheld gimbals), but none get to this level of performance. In particular these systems develop a lot of horizon drift under lateral loads affecting the accelerometer. Our system can be configured to use up to 19DoF (that's 9DoF x 2 sensors, and a barometer), which is partially the reason for the performance you see here.