The most common complication after an ankle fracture is arthritis. The goal of an Open Reduction Internal Fixation surgery is to restore the allignment of the bone and to restore normal function as well as the hope of reducing the chances of post traumatic arthritis. However even with surgery arthritis is common.
Risks and complications, as with any surgery, could be as varied as allergy to medications and anesthesia drugs, to blood clotts, to infection, to swelling, hardware failure, etc. (too numerous to list).
However, risk of not having surgery will outway all of the risks of surgery, especially since as you wrote there is "an extreme amount of displacement".

