I don't think it can be said for sure that external tibia is safer than internal tibia. External device has always higher risk of infection than internal device. External device is generally more painful but much cheaper than internal one. Femur seems to be easier and safer for lengthening and faster for recovering than tibia except for external femur which is perhaps the worst one. Regarding price, internal is way more expensive than external. Internal femur is a little cheaper (like 10-20k depending on Drs.) than internal tibia.
External tibia would has higher risk of infection, with is always easy to control and prevent by pinsite care
Internal tibia has risk of bone infection(lon latn also have) fat embolism, lower risk but when happen it is dangerous.
So I think Pure External tibia safer than internal tibia. The disadvantage is uncomfortable for long time, scars, need 1.5~2months for 1cm(always need 1 year to be normal)