Ammo compatibility in large-frame gas guns has always been a problem. In 2007, we introduced our LRP-07 and found that even some factory .308 exhibited signs of high pressure. Turns out the other .308/7.62 gas-operated AR-10 platforms on the market don't fare much better.
Running .308 through a rifle like the LRP was still doable if you chose your ammo carefully. Start running a non-standard caliber gas gun, and the problem is amplified. This is the case with any of the 6mm or 6.5mm cartridges:
These calibers do something similar to what the 6.5 Grendel or .204 Ruger do with .223. Each is designed to achieve higher velocity than the .308 while still using its approximate case dimensions. This means more powder and higher pressure.
So, why is this a problem in larger calibers but not in small-frame ones like the 6.5 Grendel?
Aside from the fact that the AR-10 never went through the rigors of military testing like the original Stoner design, it's really a matter of scale. A large-frame AR is still using the same materials as the AR-15, just scaled up.
A .223 case capacity is too small to exhibit major pressure problems. You can't fit enough powder in the case to reach that point.
Not so with a .308. Scaling up the case actually reduces structural integrity. Then, you add more powder for a bigger explosion, and you're pushing what the materials can handle.