As pointed out by Dr. Esmailian, keloids form in certain parts of the body and are more likely to occur in skin types with greater pigmentation. The risk of keloid in a lip as almost zero.
As far as lip implants go, the only implant that I would recommend is Advanta, and then only by a surgeon very experienced in its use. Lip implants are only for very select non-smoking patients who understand that the implant will be palpable to touch and may have a capacity to contract over time. Advanta success is very technique dependent, requires appropriate stretching and conforming, and requires that extension slightly beyond the pocket in which it is placed. For these reasons, most of my patients choose Juvederm or Restylane for lip augmentation.