Why the maxbotix of Px4flow is not used?

Hi guys
according to the wiki PX4flow

The PX4FLOW (Optical Flow) Sensor is a specialized high resolution downward pointing camera module and a 3-axis gyro that uses the ground texture and visible features to determine aircraft ground velocity. Although the sensor may be supplied with a built-in Maxbotix LZ-EZ4 sonar to measure height, this has not been reliable enough over a range of surfaces in testing, so its readings are not used

just out of curiosity, why the maxbotix of Px4flow is not used?
in what situation are these data false?
thanks for any answer