If any home users cut on glass, they'll be rewarded with an unpleasant screeching sound. I can't say that was by design, but it's not undeserved ;-)

The ultrasonic motion acts as an amplifier for physical sharpness. So, it's sharpest when it's got a geometrically great edge, but even as that edge dulls, it behaves sharper when on vs. off. This is reflected in BESS testing, and also in robotic cutting.

Moreover, a huge amount of the force required for cutting normal foods is actually a function of friction, not just bevel sharpness. So the reduced friction on the blade faces from ultrasonic motion remains just as effective even if the cutting edge is dull. In fact, commercial ultrasonic cutting machines don't use sharp blades at all!