Front attaching harnesses obstruct the upper arms and shoulders which can interfere with a quickly growing puppy's joint growth. Harnesses with horizontally placed chest straps tend to press on the front of the legs, restrict the natural movement of a dog's upper shoulder and can ride up to exert pressure on their trachea. X-back harnesses generally tend, for the most part, to have low attachment points that restrict back movement and exert way too much unnecessary pressure on a dog's back end. For puppies of the medium, large and giant variety who develop very quickly during their first 12 to 18 months, those factors restrict natural joint growth and bone development considerably.
Harnesses, in general, usually have control points on the forechest or back end to exacerbate delicate areas around a dog's upper shoulders and armpits as well which can be an issue and make a growing dog adjust its natural gait to avoid the discomfort. There are some excellent perfectly fitted harness options out there that address those issues and render them null and void, like martingale harnesses, however.