The Olympus E-M5 (or Olympus E-M1) are the only cameras I have found suitable/useable for the cycling videos I want to do because of sensor image stabilization for any/all lenses, plus having the right size/mass and lens selection. Specialty helmet mount requirements and still photo and yet-another-piece-of-gear issues make GoPro unattractive to me, though that might differ if it were a regular/routine use.
I’m not looking for specialty products or 3 pound rigs (I have to pedal it all up after all), just a compact system that delivers high quality 4K video. While the Panasonic GH-4 delivers outstanding 4K video and is an acceptable form factor, it has no in-body sensor image stabilization, and hence no stabilization for the 8mm fisheye or 12mm wide angle lenses that I prefer for this type of use.
My cycling videos show what I have in mind.