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.

Kilo17 can you please provide a link to Amazon/Newegg/Ebay for the powered risers that you use and stand behind? I have done alot of research and have found quite a few that seems to work, but my mining farm is virtual... Ie it just exsists in my head since my wife thinks "Saving for vacation next summer is more important than your stupid mining hobby"...

I have bought some from eBay etc but they are made differently than the ones I pictured above and are of lesser quality. Those specifically I get from Padrino here on the forum or from Scott at HolyBitcoin.com (and he was sourcing them from Padrino as well). Scott is excellent on shipping out in 1 day. If he shows no or little stock just shoot him a message because he usually has more than is on his site.