What about the permanent loss of knee range of motion often associated with external femurs, especially monorails?
There is no permament loss of ROM with external femurs - many studies exist - it's only temporary and is simply a cause of long immobilization caused by wires and half-pins that prevent knee flexion, as they go through muscles.
However, the main problem with Ilizarov femoral lengthening is the discomfort. Immediately after the surgery your knee flexion is limited to 60-90 degrees (pins going through the soft tissue) and in the proccess of lengthening it decreases to 30-60 degrees. Knee extension also suffers, but the same way as with internals, you can work it out. Now imagine having 2 huge frames around your femurs and at the same time having a maxium of 40 degrees knee flexion. Lengthening even just one femur at a time limits you much more than lengthening femurs bilaterally with internals: sitting, laying, sleeping - everything suffers.
The biomechanics thing is also true. I've read a Kurgan (Ilizarov Centre) article about it, which claimed that lengthening femurs internally, without fixing the mechanical axis, leads to arthritis in the long run. For that very reason they claimed that lengthening femurs externally is superior to internal methods. However, it should be taken with a grain of salt, as they don't have any real data to back it up. Also keep in mind that the femur angle is individual - is it known to depend on the pelvis width - women generally have bigger femur angle. The more the angle, the more your feet are going to 'move in'. Lets say you lengthen 10 cms and your femur angle is 45 degrees (in theory), that means only 50% of the length gained will transfer vertically, meaning you'll only grow by 5 cms. That also means the other 5 cms will transfer horizontally, moving your feet and altering the mechanical axis significantly. Which in the end will cause increased pressure and stress to your knees and quite likely lead to problems with your knees. Now obviously femur is rarely rotated at 45 degrees, I was just trying to explain how it works.
(I've lengthened 7.5 cms on my femur externally)