What is more dangerous surgery - BBL or lengthening in the femurs? Which surgery is more likely to get an fatal outcome?
I don't think you'll find any solid data on this, but my guess would be that BBL's are more dangerous.
The main cause of death for BBL's is fat embolisms. This happens because the procedure literally entails transferring fat from one part of the body (like the stomach or back) and injecting it into the butt. The issue here is that any fat that gets accidentally injected into a vein can cause a fat embolism. A skilled surgeon knows how to avoid this, but many people are getting BBL's from "part-time" BBL surgeons (eg. surgeons who's main specialty isn't BBL and who kind of are just doing it on the side for cash) or by less-than-reputable professionals in low cost of living practices like Mexico or Brazil.
The main cause of death for limb lengthening would be blood clots that become either pulmonary or cardiac embolisms. The risk for this is extremely low, however, as any reputable doctor will prescribe a blood thinner like Xarelto or Eliqis to prevent blood clots.
The bigger risk for limb lengthening is not death, but biomechanics. Being able to walk 100% normally, regaining full range of motion, etc.