http://www.youtube.com/watch?v=wQmlNjtiEDg
this video is best explaination
for racing fpv flyers it is recommended to use low resolution board cameras because they produce negligible latency or lag while monitoring it. At that high speed flying a 50ms latency is also dangerous and the experience will be worst....whereas for photography and slow flyers we can still use gopro,mobius etc....the lower the resolution lower will be the lag thank you
Yeap and actually all the action camera do some processing which in turn adds the latency. They use GoPro for recording but never for FPV (while racing)
Another reason to use - board cams ability to handle change in light and to see places like below trees etc which appears dark in hdcams.