My colleagues have been running Rx 580 Sapphire with 2 PSU-s x 750 watt each Modular from the Cooler Master brand. They all have connected the risers with Sata cables and they have been running from about 2 months and a half without problems.
They are using risers version 009s which I recommend you do also.
I also know people who have and still are using SATA to MOLEX connectors for their GPUs and doing that without any problems. I don't recommend doing it but sometimes it works out with using high quality PSUs They are using risers version 009s which I recommend you do also.
You can do it, but isn't safer at all. Which one safer?
- Molex to 6 pins adapter (ok)
- Molex Directly from PSU (ok)
- 6 pins directly from PSU (ok)
We can't measure it from what they do, because every card, even with the same series will have different power management if they come from a different brand.
Normally, PCIe bus will provide max 75W for GPU. So, making the same condition would be better. SATA can only provide max 54 W.