Yes it's safe as long as they are good quality.
IYKWYD you can safely use a 1 SATA adapter on some cards.
IYKWYD = If You Know What You're Doing. It means you know how to determine how much power a particular card
can draw from the slot and that it's within the SATA connector spec.
Edit: Here's a pretty good description
https://www.nicehash.com/blog/post/why-should-you-not-power-usb-risers-with-sata-power-connectorsThe only points I would add is not all versions of the same GPU model use the same power distribution
so don't go by the posted chart. Measure yourself.
Also the number of wires on the cable is a useful clue. Some cheap PSUs use only 6 wires for PCIe cables and 4 for
peripheral cables and should be avoided. PCIe cables should have 8 wires and peripheral cables should have 6.
And always follow up with a touch test to make sure the connector doesn't get too hot.