Fillers can be done on the upper eyelid for upper eyelid hollowness, which can be present due to genetics, aging-related atrophy of upper eyelid fat tissue as well as iatrogenic causes such as post eyelid surgery volume loss.
Risks will include the same general risks as with all dermal fillers, such as bleeding/vasular injury, bruising, swelling, and infection. However, this area of the face requires extra care as it is highly vascularised, thin and near the eye.
Hence it requires the careful placement of an appropriate filler in the correct plane to prevent rare but serious complications such as skin necrosis, nerve injury and blindness. It is prudent that you choose a doctor who is experienced and proficient in performing this.