If I recall correctly (Not an ME - I'm more of an electrical / software side of the robot guy) -- between the flexibility of the spline gear giving a tight enga

If I recall correctly (Not an ME - I'm more of an electrical / software side of the robot guy) -- between the flexibility of the spline gear giving a tight engagement, and the high preload / friction at the gear interface, you end up just backdriving the input if you try to jerk around the output shaft. They backlash may not be truly 0 (ask a knowledgable Mech E.), but they are definitely much better on that front than any other gearset with the same reduction I've ever got my hands on.