You can connect most risers direct to your power supply with a standard PCI-E 6-pin connection - which IS rated to handle the load VERY safely.
they are not eat 70W from sata - typically it is about 3-5W
all required power vga takes from 6x8pins from power supply
my friend had measured - and seems the system "knows" there is not enough power from risers and switches to main psu for required power
from sata comes the power to power the riser itself, that's all!
Try doing that on a GTX 1080 (180-200 watt factory TDP on most of them) with a single 8-pin PCI-E connector.
They will NOT "draw 3-5 watts" from the PCI-E bus - they will draw AT LEAST 50 for the 200 watt TDP models because they are not ALLOWED to draw more than 150 watts from the *8 PIN* PCI-E connection.
Many RX 470/480/570/580 models have a single *6 pin* connection rated 75 watts but have a factory TDP of 150 watts - those were NOTORIOUS for a while for drawing 80+ watts from the PCI-E bus (AMD fixed that with a driver change to keep the draw DOWN TO the PCI-E 75 watt spec for the bus power).
SATA power connectors have 3 12V pins (and 5 ground pins) but each pin is only rated 1.5 amps PER MOLEX SPEC FOR THE ACTUAL CONNECTOR PINS, so 4.5 TOTAL amps at 12vdc = 54 watts WHICH IS NOT ENOUGH TO HANDLE THE 75 WATT RATED POWER DRAW FOR THE PCI-E BUS SAFELY.
Forget the wiring, the connector ITSELF is not rated for the power load needed for safe operation.
DO NOT ASSUME THAT SATA POWER IS SAFE FOR A RISER CARD - the connector is NOT rated to supply the full power draw the PCI-E bus is RATED for, and many cards push that rating hard or even slightly exceed it.
Oh yeah - 1050 and 1050ti are rated for 75 watts TDP and generally don't HAVE a PCI-E power connector, though not used a LOT in mining.
RX 460 has a 60 watt TDP and no PCI-E power connector on ANY of those I've ever looked at, though again not used a LOT in mining.
GTX 750ti was generally rated 75 watts and rarely had a PCI-E power connector - and a LOT of those were used for Monero minng because at one point they were THE most efficient Monero mining GPU (and beat out most CPUs as well).
Try claiming THOSE pull "3-5 watts from the PCI-E bus" sometime....
Dual SATA to PCI-E adapter to feed a PCI-E power connector on the riser should be safe, at 108 rated watts.