I also have this board, and I am able to run 4/5 gpus' on it.  I'm also running 2x hx 850 watt gold rated power supplies.

Working GPUs:
2x gtx 970
1x 1060
1x 1080

Recognised, but not working (yellow triangle)
1x gtx 970.

Before I had the 1080 put in, all gtx 970 were working.  Now I have the 5th card recognised by windows, though there is a yellow triangle and no explanation why this card is being disabled by windows.

Anyone have an ideas about have yellow triangle? I'm going to try a third riser on the board to see if i have 2 bad ones. Seems to me that if there was a bad riser that the card wouldn't recognise in windows.

I also learned that USB 3.0 extension cables do not work with pci-e risers.