Tried it a few years ago, there were a few issues:
1. The audio quality sucks if you place it anywhere near the helmet because wind, so for insurance purposes it might be great, but for vlogging or personal use its no good
2. There have been instances of gopro's on helmets that compromise some of their safety
If I were to use them again, Id get a strap that I can use to tie around my torso, so I can get the bike's audio more and simultaneously avoid any wind noise
I also found them somewhat distracting from the ride, as often times you're checking the gopro, or worried about the battery life, and end up taking away from the enjoyment of riding itself