The flex-spline is a spring that is constantly under compression, so there is no "rattle" between teeth.
There is one downside to this inherent springiness of harmonic drives, which is some play at the end effector. I worked on a robot arm once that had 6DOF of harmonic drives and it was quite tricky to tune the controller for precise placement of tools.
Yes, and also that it's single-stage to get 50x reduction. With conventional gears you'd need 3 stages, with the backlash of the first being multiplied by the gear ratio of the other two.
Even if it was under compression against the outside ring, if the teeth were smaller than those of the outside ring, a force could cause them to rattle.
That said, if the difference in tooth counts is big enough, and the number of engaged teeth is high enough, you might get engagement from the teeth at the edges of the engaged surface, even if the inner teeth are a bit out of spec. This would cause some odd wear profiles in the teeth over time, though.
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.
There is one downside to this inherent springiness of harmonic drives, which is some play at the end effector. I worked on a robot arm once that had 6DOF of harmonic drives and it was quite tricky to tune the controller for precise placement of tools.