Hmm, true. Two of my cards weren't showing up at all. Once I did this they did show up but only one was working under windows, the other always had an error in Device Manager. I did tests swapping out extender cables and moving cards in different places, and always when I had 6 on, only 5 would work. I could remove any one of the 6 cards and the other 5 always worked so I concluded that it wasn't a hardware error. Switching to Linux was the only way I was able to get that 6th card to work.
This is with:
GIGABYTE GA-990FXA-UD7
6 x GIGABYTE R9 270X OC