My camera is for road usage, and intended as a dashcam. I don't do MTB much.
So my gopro mounts on the stem, such that it looks over the handlebars. On the road bike, the brifters are at the extreme edges of the picture so its not wide. The image suffers in the dark from being almost useless, though that's partially the older camera with a longer shutter time in low light.
- The camera points where the front wheel points. However if you're turning, its not looking around the corner where you're going.
- You can see the camera's status LED and that it is recording
- you may hear it beep when the battery runs out.
Other locations I've also had the camera mounted
in front of the head tube - too low and brake/gear cables move in front and can snag. Same goes for a fork mount - its too low to be useful. And it points in-line with the frame not the wheel
Recumbents, I have the camera mounted to the bottom bracket, which is at the extreme front of the bike. This gives a very narrow view out the front and misses anything "beside" the bike. Also it shows pedals and feet on every crank. This stays in-line with the bike frame, not where the bars point.
Chest mount - Not ideal from a crash perspective - if your chest hits the deck you'll have a hard plastic case between you and the ground. Even with a spreader plate design, this will break your sternum or ribs.
For MTB this gives a very "rider's view" angle, but tends to have enormous arms on either side of the shot. Can be hard to get a good vertical angle here too because your body moves around on MTB and on the road - a hard sprint or power effort means you lean forward so the camera will be pointing at the ground.
Helmet mount - This is probably the highest viewpoint on a bike, so camera has a top-down which can be handy. The view is aimed at whatever you're looking at, so one camera can even look backward if you do.
The weight can be significant and certainly makes the helmet heavier, and the camera can catch branches more than a smooth helmet.
In a crash, the helmet's function may be compromised by an inability to slide as designed.
Shoulder, arm, and wrist mounts. These are all a variation on the chest mount, and give you more open views on one side of the body while making it a little harder to see the other side. I'd presume you would wear the camera on your farside, (nearest the road centerline) which is the side cars pass you on.
Hand/wrist mounts let you move the camera around, though framing a shot can be hard. It will always be crooked.
Some of these niggles can be resolved with a gimbal, but that is more weight, moving parts, and added cost. And gimbals want to maintain stability in all directions, so turning a corner can upset them.
I suffer from motion-induced nausea, and there are some videos I can't watch for more than 30 seconds without having to turn away. Strangely, if its a recording of my riding, then that effect is much reduced. But views from a helmet that is moving around a lot can be unpleasant to watch.
The best viewpoint, would be a "follow-me" drone that is just behind and above you. However that's totally unfeasible on the road or when travelling. Might work acceptably in a small area, but that would give a very video-game perspective.
One oft-overlooked point is license plate capture. If using the camera as a dashcam, then being able to read plates is useful. The Gopro has too much compression and tends to blur plates in the medium distance. I had an older Kaiser Baas camera before that, which was capable of showing a plate on a car doing 100+ km/h in the other direction. However this may be better on more-modern cameras.