So I found this video of a guy cleaning their oxygen sensor out with an ultrasound cleaner. I presume what it is doing is breaking up the carbon build up in the device. Is this safe? It seemed pretty interesting.
What it is actually doing is vibrating all the crud off. I have no experience cleaning that way but I now might get one.
What is really happening is since water does not compress this is sending pulses through the water which will transfer into the crud knocking it off using physical force. might be more effective with parts cleaner or electric contact cleaner. So the only thing to watch out for is breaking the part through very high frequency vibrations, and contaminating it with whatever fluid you use. I suspect most parts would be fine but typically they are not stress tested for this kind of abuse so be ready if it doesn't work. I would be cautious with catalytic converters as well you may knock the plating off the ceramic honeycomb.
My suggestion is if you can't clean it normally and regular sprays don't work then you would have replaced the part anyway so mind as well blast it with high frequency sound, if it breaks well not really a loss. I suspect most parts will be fine though. you should also be able to use light oils too if the part is water / solvent / alcohol sensitive. the thinner the oil the better it will work.