Grounding generally is not a problem, but when I installed a B&M headlamp that was grounded to the frame with a Shimano hub that was also grounded to the frame, it did not work. And it took a few hours for me to realize that in this case I had my wires crossed. If this was my first dynohub installation, I would have looked at the paper work and made sure I did it right. But, it was my third dyno powered light setup and the first two were SP hubs which were not grounded to frame, so when I did the third I did not bother to worry about the ground, which cost me a few hours of confusion.