if I buy an ultrasonic cleaner for my records will I be able to hear the difference?

Very probably, and for the better.

But you will certainly extend stylus life. Since my cartridge is an expensive rebuild, the sonic benefits were free.

That said, I have had sealed records which sounded terrible, and could not be made right. But mostly ...

@prof 

IME, you can't judge record cleanliness by appearance. All my records have appeared pristine since I bought a 16.5 back in the Precambrian.

Five years ago I bought a lab grade German US machine and ran it at 80 KHz. After cleaning and rinsing, the records look the same - pristine. But grunge collects on the bottom of the US tank, indicating just how much further there was to go - I would say about 80%, meaning the 16.5 was good for maybe 20% of the deep crud. The sound of the records gives a similar impression.

To come clean, these are just crude subjective guesstimates, not measurements. Also, you may not get what you pay for when you buy a US machine. High frequency, high power, hot chemistry, proper spacing, and a machine that actually meets its specs all make a difference. Quality costs, here as elsewhere - but bang for buck is very, very high.

Just about anything can be hazardous if the equipment is improperly specced or doesn't meet spec.

Or appear to be hazardous after an inappropriate experiment.

Bottom line, I have cleaned thousands of records, with only the occasional warp caused by chemistry which is too hot. But then, my equipment is properly specced and meets those specs. Budget hunters, YMMV.