Camera mounts are as safe as your kayak is! It all depends on how you use it. If you're paddling and looking to film a scenic journey, any mount for your kayak camera is safe. If paddling in rougher water, however, it is important to know that long poles will probably not be able to cope with larger amounts of stress.
If a long pole hits a rock or the river bottom as you are going fast along the water, it will likely break. Despite this, there is no risk of injury to a person. Kayak cameras are regularly used for all sorts of activities, and they pose no risk to the safety of the paddler if used correctly.