Ultrasonic cleaning is the only way that can be used to wash different things in cavities or narrow spaces that are not otherwise accessible for washing. The extreme example being ultrasonic washing of seashell, with a hand brush is likely to break the seashell. The ultrasonic cleaning power is based on the miraculous power of water cavitation. That is, the washer will vibrate the washing solution at a high frequency, which will result in many points where the water pressure suddenly drops a lot. As a result of this the fluid will generate microscopic steam bubbles, which will in turn collapse due to the fast situation changes. As the bubble collapses it emits a small micro-jet that is so efficient that it can even erode metal. Examples from the fierce power of cavitation can be found in ship building industry, where huge propellers wear at an absolutely insane pace if they have been spun at too high speeds. This is the result of cavitation at its worst. In an ultrasonic washer, this destructive power is harnessed for cleaning , which in practice rarely breaks anything.
Populated circuit boards (ready made devices) cleaning can have some risk, since heavily vibration can pull off loosely attached components, break something (for example MEMS devices will break) or force the cleaning fluid inside components. On the other hand, the equipment that is usually washed is already broken, so it can’t usually go any worse.