I used an ultrasonic cleaner that used 100l of TCE at a time, and had a vapour reflow system to both dry he boards and distill the TCE to clean it. Found out that if you were more than 10s in the cleaner then parts would degrade, like rubber seals on capacitors. Leave a board in there for around a minute and the solder resist would be gone, along with most component markings. 5 minutes and the board would be bare GRP fibre again, all the epoxy gone, along with connectors and capacitor ends and bare film resistors. Drop a mechanical part like a carburettor in there ( cleaned off of most of the junk first, this was final clean) and 3 minutes later it was white. Same with waveguides and microwave parts, cleaned everything off including paint. Only plastic that survived was PTFE.
If you want to clean flux best is a dirty bath with IPA and a brush to scrub, then a rinse and a final clean in an ultrasonic bath with a second rinse afterwards then a dry in a warm area. Do not rely on the ultrasonic to remove the whole lot, it is best for final cleaning, a brush and solvent is best to get mostly clean. As you go on the clean IPA is placed in the final bath and the solvents moved to a dirtier area, so you only have to discard a very dirty cleaning solvent, or filter it and use in the same bath as it evaporates. Filtering can be done with an electric fuel pump and an in line car petrol filter, it works well, you just put 2 filters in series with the pump in between them so it only gets relatively clean solvent to pump.