To follow-up, a nice example of nbviewer being superior is that it automatically adds horizontal scroll bars for wide code cells by default whereas GitHub just cuts off your ability to view long code.
Demonstration:
Look at the third code cell here at GitHub and compare it to the same cell here via nbviewer.
If you make each window narrow, you’ll notice you cannot view the text on the far right side of the cell at GitHub. nbviewer adds scrollbars when narrow so that you can still view the code.