My results on a '15 Road Glide Special
I purchased a GoPro before my Sturgis trip last summer. Wasn't sure about mounting options, so I bought every mount I could find.
I used one of the adhesive pads on the inner fairing (under the windshield), and to that, I mounted an extension that put the camera just above the windshield. Sturdy, and stealthy, but lots of vibration that affected video and it amplified any "rattle" in the fairing. The noise was unbearable. And the only video worth saving was smooth straight-stretches of interstate. Pretty boring stuff.
Option 2 was a clamp, mounted to the engine guard. Video quality was just as bad as option 1. Lots of engine noise (not the sweet exhaust notes; the click-clack of the motor). Plus the mounting point was pretty low. Lots of fast-moving asphalt occupying the lower half of the screen.
Option 3 was "helmet cam". Best video overall. Lots of wind noise. Great footage of switchbacks and twisties. Good for studying your own riding habits. And the habits of your fellow riders (damn lane-drifters!). Ha! Even if you don't like to wear a lid, it's worth the hassle.
Starting and stopping the recording was a bit of a hassle in all configurations. Easiest on the dash. Most difficult on the helmet.
One of the guys I went with used the Apple Wifi app, constant power, and a Ram mount on the handlebar. So he was able to view what the camera sees, and operate the camera thru the touchscreen of the iPhone, instead of the tiny buttons on the camera. This also eliminated the need to charge the battery. Even with full-time power to the camera, he said it still wouldn't maintain a full charge. That was on his E/Glide, so his bars were high enough to mount the cam at a useable elevation.
I didn't try video on MY bars, as the stock position of the bars is so low in relation to the windshield/ fairing. Extending the mount created WAY too much vibration at idle, so I immediately removed it.
In retrospect, I should have saved myself $450+, and extended my vacation a few extra days. I honestly didn't expect so much vibration at all rigid mounting points on the bike. Next riding season, I may try mounting off the cowbell.