It's not a reliability issue at all.
The argument says that a carbine gas system on a 16" or longer AR is "overgassed", which apparently has some theoretical damage imparted on bolt carriers and other internal parts over long periods of heavy use. If anything, the "overgassing" makes things more reliable as it will ensure the rifle cycles with that evil, underpowered steel-cased ammo from Russia.
There is another aspect of the discussion that says that midlength gas systems allow more real estate for rails and accessories.
There is LOTS written about this, both on this site and a simple Google search, and like many "coke vs pepsi" arguments, the discussions go in the same circle.
IMHO, the 'negative' aspects of the carbine system on a longer-than-14.5" barrel are so minor that 99% of AR shooters will never see the impact. If you want the extra space up front on the rails, then midlength is a good option.