Using powered risers and Linux, they should work fine. Note that I have not used these boards. I bought them with the idea of running 7 RX 470's or 480's.

I have the Asrock H81 and H97 boards and have run 4 cards on MSI GA Z97 4 slot. I might build a rig with one...its tempting.

I would not try to run 7 cards on Windows.