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