Please remember that there are hundreds of thousands of these lenses with ring motor out there that just go on and on and on,,, Just because you see a couple of bad/broken motors and read about someone who has an issue,,, Doesn't really make me worried,,,
Not that I think the gear box design is bad it's just an awful lot of parts instead of just a few rings, as well as hear power - Speed of acquiring focus of the ring motor lenses,,,