A lot of "exposed bonded die" packages caution against using ultrasonic cleaning.

This is especially true for TCXOs, which also have the entire loose crystal in them on top of the controller die, and for MEMS mics, which are designed to be sensitive to vibration. But it's also true for things like common CMOS image sensors, which are "exposed die", but not mechanically sensitive otherwise.

Bond wires that are hanging midair instead of being pinned in place by package epoxy don't vibe with ultrasonic cleaning methods.

The risks are usually small, mind. Which is why prototyping teams and repair shops often use ultrasonic cleaning regardless. But in actual mass manufacturing, you really don't want to risk that extra 1% failure rate. So you either ask the vendors for "safe" values and dance around those energies and frequencies, or avoid ultrasonics altogether.