Unfortunately, there is no simple answer to this question. The best kayak GoPro mounts are sturdy enough to be put through their paces by professional kayakers going over huge waterfalls, however not all will be up to the task. Luckily enough, the average kayaker does not need their kayak camera mounting to go through such stress.
The sturdiness of a top-rated kayak camera mount is determined by its size, shape, and mounting system. Some of the best Kayak GoPro mounts may compromise sturdiness for a better camera angle. For example, mounts with extendable poles will be less sturdy, and may not be capable of handling an Eskimo roll. There will always be a compromise between the length of the camera mount and the sturdiness.
If you expect to be paddling in rougher waters, it is best to look for a camera mount that attaches to a gear track or mount that screws into a boat. This will always provide the sturdiest and securest mount for your boat.
As long as you know the compromises that different sizes and shapes of mounts make on a boat, you will be able to pick the top-rated kayak camera mount for you.