A most interesting thread. Where I work these days, some specialised electronics assembly is done according to ancient lore laid down by folk long gone, and I am deeply dubious about some aspects of it. They sometimes put PCB's in an ultrasonic bath filled with some kind of thin citrus-smelling liquid for 10 minutes of shrill abuse.
It doesn't seem to clean flux off much though. And I wonder whether the ultrasonics could damage stuff (I asked if it could be used on a device with exposed wire bonds, and the manufacturers said definitely not.) I would be interested in information on that. Meanwhile, I'll give RMA flux and acetone a go.