Are surgeries for tumors in the brainstem particularly dangerous?

There is more risk. The brainstem has a lot of important function packed into a small area. Surgery there has more risk associated with it than surgery in many other parts of the brain.
Yes. No surgery is without risk and brainstem surgery is no different. The brainstem is a major relay center for information going to and from the brain. Having said that, there are safe zones that one could use to enter the brainstem.