Risks associated with lift and implant performed together
A lift with an implant is controversial for two reasons. First, when you perform a lift you are making everything tight and closing the wounds under tension. It you add the expansive forces of the implant at the same time, you are fighting against yourself. There are forces on the wound which try to make them separate, which results in wider, thicker, more irregular scars. In the worst case, the wounds will open. So compromises are usually made in the operating room by the surgeon because they cannot close the lift wounds over the appropriate sized implant. Either less of a lift is performed so that the skin is not as tight and therefore there is less tension on the closure. Or a smaller implant than would be appropriate is used so as to decrease the expansive forces. Either way, you are compromising the aesthetic outcome. Often the outcome is so compromised that a second revision surgery is required. If however, you plan to have the lift first and then the augmentation after everything has healed, then you have two operation that are planned, both with much lower risk than the combined mastopexy/augmenation. The outcomes of the two meticulously planned operations are much better and a more aesthetically pleasing, and a safer outcome is achieved.
The second reason the combination of mastopexy and augmentation is controversial is because of the risk of nipple necrosis (death of the nipple). By making the skin tight for the lift, you are putting external pressure on the veins that supply the nipple. By putting an expansive force on the undersurface of the breast with an implant, you are putting pressure on the thin walled veins that supply the nipple. If the pressure by squeezing the veins between the implant and the skin is greater than the venous pressure in the veins, the flow will stop. If the venous outflow stops, the arterial inflow is stopped. If the arterial inflow is stopped, there is no oxygen for the healing wounds and the tissue dies.
Placing the implant on top of the muscle in combination with a lift puts the blood supply to the nipple at a much higher risk because in addition to the issue of pressure on the veins, you have to divide the blood vessels that are traveling from the pectoralis muscle directly into the breast (and to the nipple) in order to place the implant between the breast tissue and the muscle. This adds a third element of risk to an already risky operation. Mastopexy/augmenation with sub glandular implant placement is by far the riskiest way to address your anatomic question.