I see many very good watchmakers using agitation / rotary drum cleaning machines, but I'd have concerns about the effects on the hairspring for these out of the ordinary forces. Also the tendency for the smallest screws & cap stones getting cast about & wedged into some little crevasse in the parts cage.

I normally use an ultrasonic bath (Zenith 67 cleaner / Drizebrite rinse), but are there downsides to this that I'm not considering where an agitation system would give better results?

 

I've cleaned thousands of movements in a machine that spins the baskets (both during cleaning, for spinning off the excess fluids between jars, and during the drying cycle) and I've never had a balance spring damaged or affected in any way. This concern is very overblown, as pretty much every watchmaker and watch manufacturer uses some form of a machine like this.

Well, most people don't just throw small parts into the larger basket - they go into smaller baskets with lids to prevent the exact thing you are afraid of...



Note that agitation can come with ultrasonic or not, so it depends on the type of machine you are contemplating. One thing with ultrasonics is that you do have to be careful how long you leave the parts in there, because too much time subjected to ultrasonic cavitation can damage the finish on plates and bridges, stripping the plating off.

 

I clean with ultrasonics, using L & R Extra Fine cleaner, then two rinses with L & R # 3 rinse in a centrifugal machine, then heat dry in a centrifugal machine. Anyone who loses track of small parts, or has hairspring problems, needs to refine their processes. Bergeon mini-baskets work just fine.