Is it better to have fat transfer into the muscle of the buttocks or only superficially?

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 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 has convened 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 and 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.

Thank you for your questions. The Brazilian Butt lift procedure carries higher risks compared to other cosmetic surgery procedures. The biggest risk for a serious complication is when fat is injected deep into the gluteal muscle which could lead to a fat pulmonary embolism. 
There are steps and techniques your surgeon should use to minimize this risk. One technique is to keep the fat superficial to the muscle.  The size of the cannula and the direction of the fat injection are very important.  
Make sure your surgeon is a board certified plastic surgeon so you will be confident in their abilities and dedication to safety. 

I put the majority of the fat into the subcutaneous tissue. The concern about placing it into the muscle especially deeply is that it may lead to injection into the large vessels which may lead to a pulmonay emoblism.