How risky is a retinal detachment surgery?

Not very: The techniques have been well worked out for retinal re-attachment. But recovery depends upon the extent of the detachment, the cause of the detachment and whether the central vision is involved. It also matters whether you have had previous surgery on that eye. Discuss with the retinal surgeon prior to the surgery.

Depends: No surgery is without risk, but most do fine. The bigger risk is not getting it done if indeed you have a retinal detachment - then you really are risking blindness.