I should have prefaced by saying this only works with carbon steel. As others have said, the quality of steel makes a difference. The sdp-si gears seem to work well and doesn't rust afterwards. However, the traxxas gears I've done rusts within a day or 2 just exposed to air. They are probably using cheap steel, but the surface does seem harder (using the file test) than before hardening.

The hardened layer with this method is going to be much thinner than if done in a factory with a controlled environment. The way this works is that the powder creates a carbon rich environment as you heat it to hardening temperature. The more carbon in steel, the better it is for quench hardening. The longer you heat it, the deeper the carbon penetration and thicker the hardened layer.

I would think things like dogbones and drive cups could be hardened as well, but haven't tried. Would hardening the spur and the pinion be beneficial? As with anything, experiment and your mileage may vary.