Back in high school lab I remember we were told not to put our hands into ultrasonic cleaners because it messes with your bones or joints or something like that.

Was that just bullshit?

They probably wanted to keep you from making a mess. (And perhaps also wanted to mess with you.)

It depends on the frequency, intensity, and duration of the ultrasound.

Which probably aren't documented for the ultrasonic cleaners.

Plus "if you don't know, play it safe".