Actually head mounting is a lot more stable than bar mounting, at least on the roads I ride on.

There's so much vibration on the bar or frame that as long as I'm moving, everything is very blurry. It only becomes sharp when I stop moving. On the helmet I look around some but it's not nearly as bad as on the bike.

I've found that the stiffer the mount, the better. If there's much give at all in the mount, the camera footage looks like it was shot from a paint shaker; you can't see a damned thing and it's hard to even tell what's going on.