I let go of my paddle once a few months ago. I got pounded and the paddle seemed to twist around and I could not hold on. But the difference between kayaking and SUPing is that it is easier to get back on the board and prone (or knee) paddle after the paddle. As Colas says, the next challenge is finding it. In my case I caught a wave and rode over the top of the paddle?

?which damaged the blade...
It looks like a good design though. If the length of the tether is correct then you won't accidentally grab it when changing sides - that's what I would fear.