Neat. And it'll get rid of the mice and bats in your house.
But seriously, I wonder how much acoustic energy - that they point out you can't hear - is still entering your ear and interacting with the delicate structures there.
Frequencies you don't hear - can't hurt you, or rather they cannot damage your hearing. Sure, submerging your head into an ultrasonic cleaning bath will irritate your skin, but the energy won't vibrate away your hearing abilities, if it had the potential to do that you'd hear it as a loud unpleasant sound.