DA Polisher which throw? 15mm or 21mm
Specifically the OPTiX 15mm is a much smoother unit, and overall a better user experience over the 21mm. The 15mm, 12mm and 8mm DA polishers in the OPTiX range are smooth and compares quite nicely with the more expensive brands. The 21mm unit is a not comparable to the big brands like the Rupes 21mm. The Rupes 21mm DA is a noticeably smoother machine over the OPTiX 21mm.
All other units of DA OPTiX polishers from the the 8mm to the 15mm are all very similar in a great user experience and same if not better performance.
I have just run the machine as part of our checks, I have removed the backing plate to double check the 4 screws that hold the counter-weight surround.
It is important that you check the tension on the backing plate after a few uses as well as the 4 screws under the back plate (you need to remove the backing plate to see the 4 screws).
Dual action polishers naturally vibrate a bit that is part of being a DA, the vibration can loosen parts over time. If it concerns you I suggest buying thread glue.
Other known problems is brushes, there is a spare set in the bag. When things just don't sound right or speed is doing funny things then the brushes probably either need replacing or just removed cleaned and put back in. It is not usual to have brush problems til after 100 hours.
Hope this helps