Dwell time is less on a mid length which is a good thing, gas is tapped at a lower pressure as well. It's slightly softer shooting and is what a 16" barrel should be to keep the same dwell time as 14.5" carbine gas and a 20" rifle gas.
Longer sight radius, more real estate when using a fixed front sight are also advantages.
As long as it's ported right I see no disadvantages, at least in a 5.56
