It's not RainX. It's a design and part problem:
The reason is improper tolerances on the wiper bracket. If they are slightly loose (which they are) they will wobble until the keep making the bracket even larger, and wobbling even more, and repeat. My solution was simply to use a small C clamp to press the metal side of the bracket until there was a solid connection to the wiper side of the bracket. No more wobble. For now.