Yes, I left the media question open as possible downsides may vary with media.
My main concern was with the disruption to the surface treatment of the teeth leading to premature wear and hence whining, or to even cause the teeth to whine more than they would otherwise.
Also, the possible embedding of particles of media in the surface of the gear leading to premature wear of not just the teeth, but the whole gearbox.
Heavy mineral media like garnet is terribly abrasive, even in it's finest powder form.
Certainly you would want to stay clear of bearing surfaces were needle rollers run.
The reason for asking this is because my nephew is building up a box which needs a cluster gear and also had concerns about the ebay offerings.
btw, Rob - not Ron