Advise me about ultrasonic cleaning of bike bits.

If the ultrasonic cleaner removes any internal grease from the mech, how would you replace it? 

I occasionally clean my chain in an ultrasonic cleaner and it does a good job of removing the fine grit that would otherwise be hard to clean off with an old pair of underpants! 🤔😲

What do you mean by clean? And what does a brush and hot soap water missing that stops the mech functioning or looking clean?

Ultrasonic cleaner will do a brilliant job but add loads of faff - removal, reinstall, getting the cable tension right.

Personal choice but that seems far too much effort...and would likely mean other bit and parts then looked out of place and would lead to more (and then more) bits needing removed, cleaned and reinstalled.

There's a bunch of good stuff on YouTube on these. Eg this one from GCN.  

Or this one

In the end I concluded that the size of machine needed to clean a wide range cassette was too much for the amount of use it would get.  

I would not put a mech in one due to the internal grease getting removed and being hard/impossible to replace as already noted above.  

I tend to clean mechs with some soapy water or general bike cleaner and a long bristled paintbrush and toothbrush.  Jockey wheels can come out from time to time. 

Cassettes come off the bike for a major clean or get the muc-off / foaming cleaner and soft bristled dustpan brush treatment.  

Chains can be split and given the liquid degreaser and mayonnaise jar treatment.  

Having said what I did I'm just watching a video where the mechanic has just chucked two mechs into a parts washer! 

I can only assume you mean the jockey wheels, otherwise I don’t see much point in cleaning the actual body of the rear mech. In which case SRAM mech and lock it to remove chain tension then a toothbrush and whizz it round. Once every 12 months or so remove said jockey wheels and the arm for some brake cleaner action.

I tried it a while back with an  Aldi US cleaner. Came to the conclusion it was more grief than it was worth. 
You need quite a lot of cleaning solution (I used white spirit) to cover the parts. You then have a pint of so of mucky solution afterwards. So you leave that to settle for a bit, and come back to it a few days later to carefully decant the cleanish stuff, leaving 1/8” of black gunge that needs wiping out and disposing off. All for a marginally cleaner chain, bearings or jockey wheels, which is about all they’re good for.

I may have a different view if it was a full time workshop a bigger unit and in use multiple times daily…

Cheers for the advice one and all, I have now been suitably dissuaded. 

30 years of cycling and its never occurred to me that there is a need to do this. A quick brush, hose and a few drops of lube then crack on. Detailed cleaning feels like a solution to a problem that doesn't exist for people with too much time on their hands! Bike are for riding, not for 'detailing'.