Nice warning about those kind of SATA cables. I burned a cable like that when I connected two RX480 cards to a single one of those. But the cable quality wasn't really great to begin with. The wires looked thick enough but the connectors were really loose ("pressured razor blades" kind of fitting in a lazy fashion). Although, 1 card works just fine if the cable quality (including the connectors) is decent (and you don't try to win overclocking contests).

I thought about modifying the risers like that but I ran into a false problem. I though: "nahh, the cards might need the 5V lane, not just the 12V". But I was stupid because I could simply leave the 5V wire as is (on a SATA/molex) and only cut/redirect the 12V. Although, as I said, 1 card per decent SATA cable works just fine for me, so I will leave it be. (But I replaced that PSU with one which has better SATA connectors. The old PSU could be hanging on the balance with 1 card.)