I have recently started running on sand but i had people telling me that running on sand for long term causes grave injury knees and joints.
The things you've heard can be true, but it's not 100% sure that it will happen to you. How long do you run? It's possible to hurt your ankle and knee if you step on the sand under the wrong angle. Your knee cup might be hurt also. That's why running barefoot is not recommendable. Rather use some really good sneakers to run with. That way you will protect yourself from injuries.