I suspect that your question actually concerns the physical effects of the ultrasonic waves, but even before you get to that stage, you should consider the simple chemical aspects of immersing a PCB. In the past, ultrasonic cleaners often made use of solvents, and in particular, the Freon compounds. But Freons were banned and then there was a strong general push to move away from volatile organic compounds (VOC's), so that now, ultrasonic baths tend to be used with water with a cleaning additive. Additionally, it is often the case that the recommendations are for the bath to be run somewhat above ambient temperatures. So, even without considering the ultra-sound, you now have a nice cocktail for corrosion, especially so when different metals are present which may encourage galvanic corrosion. You'd need to proceed with caution; I've seen some quite weird things go on in ultrasonic baths.
You can use propanol (IPA) as a cleaning agent, though it not particularly effective and it is quite volatile.
As for the effect of the sound waves, I'll leave others to comment.
S