Powered USB risers are infinitely better, safer and likely even cheaper. You don't have cards heating each other being close, you don't risk burning your motherboard's 24 pin socket because the cards pull too much from it and you can have 6 cards per rig if you go with something like an Asrock Pro BTC board.
The issue is I already have a large gpu hanging rig with 8 amd cards, I wanted to make a small case rig since these stay so cool and use so little power, does the extra 8 pin power on the motherboard I linked not help with this?
I see. I guess it does, however I'd only use cards which also have 6-pin sockets on them so they would draw most of the power from the PSU itself, instead of pulling up to 240W from the board.
An x1 PCI-E port can output 10W, a x16 PCI-E port can output 75W, same for 6-pin and a 8 pin can output 150W.