Even if you have proportionally unusually short tibias/femurs compared to the other bone doing any significant amount (i.e. more than say 3 cm) will knock your natural biomechanics out of whack and put you beyond the natural ratio for proportionality. This does not mean you'll look aesthetically bad but your body will have a hard time dealing with it and your might develop problems like arthritis in your knee joint later on if your tibia is proportionally long for example.
There's also the case of soft tissue stretching. Whatever amount you will do you will damage the soft tissue but that can me mitigated my smaller amounts of lengthening- sub 5 or 6 cm. But whatever you do no, it cannot be 100% again once you have lengthened, because it will always be stretched. You can mitigate that damage by lengthening a small amount, doing physiotherapy, not legnthening too fast, and stretching well before surgery and during and after the lengthening phase, but you can still never be 100% again (i.e. for athletic capability or potential muscle strength).
Then there are general aches and pains, poor union, nerve damage.. none of those things are inevitable but there is a big risk, and you can't discount the risk when you make this decision because it can happen to you too of course.