I suspect the guy on the S&W phone just told you not to use ultrasonics on any thing to avoid having to read off a long list of guns it is safe for and the newer models with special finishes it's not safe for.

Some of the newer guns have specialized finishes that seem to be sensitive to solvents and cleaners, and ultrasonics often make things worse.

A big advantage of ultrasonic cleaning is that you don't have to disassemble anything.
We watchmakers use ultrasonics to clean some small components to avoid having to disassemble them.
If an ultrasonic cleaner can get a partially assembled ladies watch movement surgically clean, it can sure and hell get a comparatively gigantic gun clean without disassembly.