You have to be very cautious with any item that has a painted on finish ... for example the green glass in the above shots. A lot of glass has not in fact been tinted but has a film of colour on the outside. The ultrasonic vibrations will strip it off very efficiently. Same goes for paint on cheap clone atties, also for the expensive, non-reflective finish applied to spectacle lenses.
O-rings are not a problem at all, though it can't clean under the o-ring where it's in contact with the atty. For the best results it makes sense to tear down completely and rebuild/reassemble afterwards ... but for normal cleaning purposes you can leave o-rings and grub screws in place. Loose grub screws can become unscrewed by the vibrations and fall out, so always check the water for tiny items before you throw it down the sink.
What it's really good at is cleaning complex coils, like fused claptons for example.
Actually, the most pleasing result I got was from a manky can opener ... you know how grubby they get and how hard it is to clean them up? It came up like new!