For example, in the case of an eyebrow implant, the incision can be made at the top of the forehead behind the hairline. Other than incision placement, incisions should be made with the grain of the skin, and should be treated gently during healing. Implants can be stretched just like piercings. This process is repeated to achieve the final size. There are some risks of irritation to the skin above the implant if this process is pushed too fast as with all stretching.
Infection is a minor risk that can be minimized by proper technique.
Reaction to the material can also be a risk, but assuming that the subject is already aware of any materials allergies, this shouldn't be a problem. On an aesthetic level, rejection and migration can be a risk, but again, this can be minimized by proper technique. Assuming that the implant is placed sufficiently far from the insertion point and placed deep enough in the skin, migration should not be a risk. A pressure bandage holding the implant in place may help for the first few days, as will taking care not to sleep on the implant pushing it out of place. In any case, some degree of migration should be expected.