[hal9000]

In terms of the ultrasonic damaging an aluminum suppressor, they really shouldn't. First of all, most modern ones are adjustable so you can tune the frequency they work at. Second, most are very low power. You can immerse you hand in them with no pain or damage (it will clean the grit out from under your fingernails though).

I could see an ultrasonic damaging a poorly applied finish, or if you just drop the silencer in, maybe scuffing the outside finish as it vibrates against the vessel. You should always hang items in a plastic lined basket or by a wire in the cleaner.

One other possibility might be residual cleaner or moisture causing corrosion on the inside of a suppressor after the fact. Aluminum does strange things when left in contact with water for long periods of time.

That being said, it would be worth doing some testing before hand if you ever decided to use one.