Is it safe to use an ultrasonic jewelry cleaner on an Apple Watch?

The second paragraph in Cleaning your Apple Watch - Apple Support says

Clean and dry your Apple Watch

Keep your Apple Watch clean and dry. Soaps, cleaning products, abrasive materials,1compressed air, ultrasonic cleaning, and external heat sources can cause damage and shouldn't be used with your Apple Watch.

‘Nuff said.

Apple does not point to exceptions in the caveat. If you damage the loop cleaning it with anything but what Apple suggests you will need to take it up with Apple if you require a repair. I have a trampoline in the yard, it does not imply I should use it to clean the gutters on the 2nd floor.

Your question was asked and answered, despite not being answered with what you are intent of hearing. If you want to clean some part or all of your "watch" with method Apple is stating not to and you have to bring it in for repair Apple can likely void your warranty on the spot. So proceed at your own discretion.

Secondly; that metaphor is bomb yo.

I wouldn't use it on anything you mention.

I cleaned my silver milanese at a jewelry store before giving my Series 0 away, came out fine. I wouldn't do any but the all metal that way.

No. I would not use it on an iPhone.

NO and don't even think about it again.....