When I talked to the guys at Criterion they made it pretty clear that the Core series porting is biased towards suppressed use.
I can't answer why people have so much trouble with stuff, other than that people will screw up things in ways you never thought possible. Most people seem to have no diagnosing skills beyond complaining on the internet.
I've been using multiple RLGS 16" barrels with no trouble for years. I've got a RLGS 14.8" that's been flawless, and I tried to make it fail because I didn't believe it would work. After it worked great I had a 17" RLGS +1" made just because "it can't be done!" Ran flawless all last match season, even used as a loaner a couple times and fed all kinds of crap ammo.
Trust me, 18" RLGS barrels are not difficult to get running perfectly and they're nowhere near the ragged edge of function. Any gassing issues can be fixed by 2 minutes with a drill bit.
Also, AGB's control gas volume, not pressure. You can't get the same effect as a long gas system when you still have the higher port pressure of a shorter system.