BBL Fat Transfer into Muscle, how worried should I be about this technique?

Fat embolus is a very serious and known complication of fat transfer (which has been recently spotlighted in the media). While it is true, injection of donor fat into the infra-muscular plane (within the muscle) increases fat viability this does come with a significant increase in risk. There are large vessels deep to and within the muscle which can be inadvertently injected while grafting leading to a fat embolus. The consequences of such a complication are very serious and life threatening.
At the recent American Society of Plastic Surgeons Hot Topics (2016), data from AAAASF (ambulatory surgical certifying body) was presented, suggesting that BBL/gluteal lipoaugmentation demonstrates a dramatically higher risk profile than other surgeries. The Aesthetic Surgery and Educational and Research Fund is convening a task force as well. The hope is that we will have more concrete guidelines for addressing these very serious complications.
That being said, the safest way to avoid fat emboli is to stay in the subcutaneous plane (at the expense of fat viability)and avoid the deeper muscular plane. If grafting of the deep plane is planned, surgeons should consider blunt cannulas, aspiration prior to injection. Using a supra-gluteal approach to minimize the risk of inadvertent intravascular injection has also been proposed. While this will likely impact graft viability negatively, it will also lower the risk of a serious complication.
With regards to your specific question, according to AAASF data the risk of death from BBL will range from 1:3000 to 1:6000 (subcutaneous grafting will reduce this risk relative to intramuscular grafting). By comparison the risk of fatal complication from abdominoplasty is 1:13,000. It is unclear how the relative risk falls with subcutaneous injection vs. intramuscular injection (however, in all likelihood, it is less).
In summary, the procedure can be safely performed, however, they should be performed by a board certified (ABPS) plastic surgeon with experience. These risks should be discussed as well as the strategies being employed to avoid them.

There is a lot of research and controversy about the proper technique for a Brazilian Butt Lift.  You are absolutely correct about being concerned first and foremost about safety.  Injecting into or below the muscle can result in serious complications such as fat embolus.  I do not do intramuscular or submuscular injection due to safety concerns with these more aggressive techniques.  

Hello,
if The doctor (plastic surgeon) know the anatomical gluteal area, can reduce the risk considerably, to avoid vascular injuries and future complications.
Good luck,
Dr. Francisco Rodriguez, MD