Yabba 1.1.1.10
FTBLib 5.0.1.15
FTB Utilities 5.0.1.12

I'm using an Item Barrel Connector to import from a ME Storage Bus on a wall of barrels, and using Compound Barrels to turn the corners of the walls. When I place an item that doesn't have a barrel into the ME system, it is stored in one of the Compound Barrels. It appears to store more than a stack of the item, but I haven't dumped enough items to reach a limit (25 stacks without hitting one)...so I'm guessing the Item Barrel Connector is somehow seeing is as a normal barrel.

My understanding was that Compound Barrels were not supposed to be able to store anything, and just connect groups of barrels around corners/walls...so I'm reporting this as a bug.

Also noticed they don't seem to "connect" Antibarrels, not sure if that's intended or not.

Nope, compound barrels are meant for storing items, but they dont have a model/skin and they cant have upgrades (except for tier upgrade, if not in this then next update). Essentially if you have a huge storage storage facility and you dont want to lag the hell out, you use compound barrels, because they render as fast as normal blocks, like planks do.

Antibarrels are not meant to be connected, mostly for code reasons. But also so you can connect it seperately with ME/RS and set its priority to higher number than other storage places