The bolt and cycle of the AR15 were designed to work with the 20" rifle. On a 20" rifle, the gas port pressure is around 12-15k psi and located about 13" down the barrel. In a carbine, the gas port pressure is about 28-30k and is located about 9" down the barrel. This means that the carbine cycles harder, faster and is trying to extract the fired case while pressures are higher than they are in the rifle.
A midlength drops port pressure to around 18-21k and moves the distance out another 2". These are pretty well understood facts. Just shooting a carbine and a midlength side by side you can tell the difference in the smoother recoil of the midlength. From a practical aspect, this should mean that lifespan and parts breakage on a midlength should be more rifle-like than carbine-like. Just watch the training video of the XM16E1 and you should be able to understand why the amount of gas pressure and the distance it travels is relevant to the functioning of the rifle.
Also, while a midlength isn't exactly standard there are literally dozens of firms making midlength furniture, so parts shouldn't be an issue at all.
Based on what I have read, 16" barrels with rifle-length gas systems suffer from some of the same short dwell time and reliability issues that plague the 10.5" shorties. With so little barrel past the gas port, the bullet leaves the barrel and the pressure drops before the gun can get enough gas to cycle reliably. If you go this route, you need somebody who knows how to open up the gas port to the correct diameter and a muzzle device that creates some backpressure isn't a bad idea either. However, a better response is either a midlength or carbine gas system under the handguards.
17" is about the shortest you can go and use a rifle-length gas system without adversely affecting reliability.