There is a recent study on two hazards of bully sticks: empty calories and bacterial contamination. Note: The bacterial contamination is more of a health risk to humans than to dogs.
Tufts Study: 'Bully Sticks' May Be Bad For Dogs, Owners | The Grafton Daily Voice
And if you give your dog a bully stick, you also have to watch to make sure your dog does not choke on or swallow large fragments. I would never give one to Bella, but many on Yorkietalk swear by them. You can get them at Petsmart or online.