I read about fried Asrock Pro BTC boards when people used them with powered risers but also heard claims that without powered risers only 2-3 cards would work.
I'm planning on building a rig with an Asrock Pro BTC mobo in mind with 5 cards, but with 2-3 types of video cards, with different power consumptions. So what would be the safest route to take?
Always use powered risers and if you don't understand what I just wrote, let me put it a different way..............................
ALWAYS FREAKING USE POWERED RISERS!
There!
Get it?
Got it?
Good!
My $.02.

Not for the asrock H81 Pro BTC motherboard. It provides supplemental molex power to the PCIe ports already, so powered risers would be redundant and likely to cause a short circuit as described in threads in the past.
What?
No, just no. Powered risers will never cause a short with the pci-e lane, no mater if the bus has supplemental power or not. 12V is 12V.
I'd go a step further and say never use ribbon riser cables, but if I were to use those pieces they would certainly be powered. LostDuchman knows what he is talking about, use the powered ones no matter what mobo you use.
My $.02.