I roof m0unt mine. Running on dirt, the vibration of the screen really took away from the picture when dash mounting, plus you can see lot more of what is going on around you when the camera is outside. Flipped the car and ripped the camera off the roof, and it still kept working
The idea behind the rule is exactly that, you can see and record everything, so in theory the camera is being used as a data acquisition or telemetry device, which is illegal, therefore mounting the camera where driver input and gauges are visible illegal. It is a great tool to improve your driving
I don't write the rules, just follow them