My friend is running a rig with 4 RX 480 8GB Devil after market cards with the USB 3.0 and he has no issues. This tells me it's the quality of the risers you purchased are junk!
I have over 250 GPU's running and have been mining a long time. I will be happy to show you a BIG BOX of worthless USB risers. Every brand, color, manufacturer. Secondly, If your friend is running some 480's it is probably a new build - give it some time.
