depends what it is that needs cleaning
if you think it would work applied manually and it won't damage the item its fair game....
i have used all sorts
standard parts cleaner or parafin
good for oily varnished nasty bits of car
although the kind of disinfectant floor cleaner/degreaser/polish used in schools and hospitals works quite well as well.
it tends to degrease without the use of an oil based or thinners based action i.e the thing ends up grease free but gritty
for records i used deionised water and isopropanol 70:30
i have used tap water and washing machine liquid. foams less than washing up liquid i.e for things that would normally see that kind of stuff
if the thing doesn't mind alkali soda crystals or diswasher tablets can work, this can make stainless get a kind black tarnish or will eventually turn aluminium or pot metal a darker grey
the main gist of it is leave it for a long time doing its thing, its not a "Right before Your Eyes" magic cleaner. it takes a while...
you are using it to clean the bits you can't reach or are too convoluted or down a blind hole, to get access to, with your favorite bristle brush
avoid drilling out all the little lead plugs in a carb body... etc etc
main thing to remember is the ultrasonic action heats the cleaning fluid all by itself so if using something volatile or flammable best place for the work to be done is outside, you don't want to stink out the place or blow yourself up.... check it every hour or so for temperature you don't always need the in built heater. use that if the solvent is water based only....
nice-clean, parts-washer fluid from machine mart works well on spark plugs...
but any bulk packaged unbranded brake or carb cleaner off ebay would also work
i.e this cheap stuff
https://www.ebay.co.uk/itm/383640942414 ... Sw6DhdoIZx
or a big tin of thinners
if usuing indoors stick a box over it. the buzzing will invariably annoy one of the familiy

and its all YOUR fault....etc etc etc
i tend to look up the "magic in a bottle" on google. find a US brand that cost a fortune and is hard to get here
thn look up a uk equievlent and you often find similar is made by some factory in the backend of nowehere in the UK and sells for few £
the other option when nothing works is TSP
its kinda nasty stuff but Boy does it clean its alkaline it can also soften/remove paint its not good for you or the environment...BUT get used in dishwasher tablets decortaors wall and wood work cleaner is used as a food additive..... just be careful.... we are not talking caustic soda wrongness or hyrdrocloric acid wrongness its just not very nice treat it like you would old petrol or used engine oil...... i.e wash hands and don't imbibe it, breathe it... or let it seep through your skin.
https://www.activemyhome.com/what-you-m ... -with-tsp/
i used it for removing grout mildew mould and the kinda greasy gunge tennets oooze all over everything when i had a rental flat.
its the landlords fall back to remove traces of the last lot who didn't pay their rent.
will make white spots on your jeans and give you sore hands........!
Dave