Do these allow current to flow back to the motherboard? That's an issue I've seen with a ton of these risers, which is easily resolved by modifying the ribbon. With these, I would not be able to make that modification if necessary. A ton of motherboards are being fried by the board being overloaded by current flowing back from the 12v line.
A ton? Show me links to that. Motherboards get fried when people run five or six gpus with unpowered risers each trying to draw 2-3 amps or more. Do you even understand how a powered riser works? There is no "current flow" back to the motherboard. The USB lines handle data, the molex provides 12v and 5v (converted to 3.3 for pice spec).