I would love to see any examples of patients or scientific proof of femur LON being more dangerous than tibia LON. Yes whenever an external device is involved (no matter pure external or LON), surgery on femur is more painful, but in which way it is more dangerous or risky than tibia? Why in world it should be? In fact, femur bone tends to consolidate faster than tibia, and this is a plus. And not everyone can lengthen tibia because some people already have long tibia/short femur. There may be methods like Precice 2 and Stryde. But does everybody have budget for this? LON is still a good option for many patients who are tight on budget. The internal nail shortens the time spent with external devices, and it stabilizes the bone too (preventing misalignment of bone).
For people who don't have the money for internal methods, are brave enough to tolerate pain, and take care of themselves (pin site cleaning, physiotherapy, overall hygiene), can consider LON on femur. I am yet to see someone who proves the opposite.