IIRC The like for ex 500 teeth mating with the 501 teeth other side do make them indeed 0 backlash. but if that was all and they are 'better' than planetary boxes as your title asks, why have they not replaced plannetaries? there are reasons that go beyond pure backlash. one that prevents me (and I am sure others) from considering them is their inherent ripple: velocity output = velocity input * ratio * cyclic ratio that causes the output to no longer be constant speed. true it is small ripple but apparently large enough to show up in parts machines - unless you go with external scale feedback, in which case you MAY get rid of it if your velocity loop is high enough bandwidth, which it may not be at high speeds..... most of the time the 3 arc min backlash of a planetary will do the job, or the extra cost down to 1 arc min. we even use dual motors/planetaries/pinions on racks to eliminate backlash electrically rather than use one of these....