What are the risks of surgery to treat chiari malformation?

Many things. Surgery to decompress the herniations can either focus on removing some of the bones to allow more room, or detangling the spinal cord as it leaves the brain (rarely done). This is serious surgery that can result in infections, paralysis, problems breathing, and worse. However neurosurgeons today are incredibly trained and can do amazing things!
Ask ur Neurosurgeon. Many of the complications of decompression surgery have to do with opening the dura and research has shown that opening the dura does increase the complication rate.  there is a risk of infection and sometimes the patch that is sewn in leaks or becomes scarred.  the brain may slump further into the spinal area in the postoperatory or later.