How serious or dangerous is the surgery to correct scoliosis?

Major surgery. Scoliosis is major surgery. In experienced hands it is safe, but all surgery has risks. Depending on your age, health, curve location and severity, the risks can include bleeding that requires transfusion, infection, nerve damage (weakness, numbness), residual or new deformity, implant failure, medical problems, even death. Talk to your doctor about your situation. Consider a second opinion.
Very. Blood loss can be dramatic, even your entire blood volume. There is also risk of nerve damage, which intraoperative monitoring helps protect from. Risk of blindness from the surgery is poorly-understood, but related to blood loss and prone position. It's major surgery in every way. That being said, it can allow you a much better life than without.
Very serious. Any spine surgery, and scoliosis surgery in particular, is major surgery and carries a real, though small risk of permanent nerve damage or paralysis. For patients that are considering this, make sure to find a surgeon who will take time to educate you on all the potential risks and benefits prior so that the patient can make a fully informed decision.