Uncommon. If they were common, that surgery would not be performed. That said, the risks really depend on which sinuses are treated surgically and with which surgical technique. Overall, it has become a low risk surgery. Talk to a surgeon.
Safe surgery. When done for the right reasons for chronic sinusitis, sinus surgery is 90-95% effective at reducing symptoms. Risks are low but include: scarring, anesthetic complications, bleeding, infection, recurrent disease, pain, voice changes, reduced smell, and nasal obstruction. Extremely rare complications in skilled hands include injury to the eye/brain injury, CSF spinal fluid leak, blindness, and death.