Some people don't like knee boards because you have to move your head up-down-up-down to use it, and that can be disorienting, especially in poor visibility conditions (when external visual references are minimal). There are other types of mounts available, such as suction mounts, that mount the device closer to eye level, but while robust these are not as secure as built-in equipment. The overheat problem is real as well, but having your iPad shut down in flight is nowhere near as big a deal as having it jam in the rudder pedals.
Further reading: https://airfactsjournal.com/2018/10/how-spatial-disorientati...
