The other day i was cleaning my tank, My ray was under a piece of driftwood. I bumped the wood with my Siphon and watched in astonishment as the Ray whipped his whole body upward at the wood. If that was a hand there would definitely be major impact. So for this reason i dont go trying to shove my whole arm in the tank if its not completely necessary.
On the other hand if i hold a worm at the edge of the tank with my hand still out of the water he comes up and takes it from me. I consider this pretty safe and that is my method.