Is it electrically safe to use an ATX PSU's PCI output for SATA/MOLEX instead of graphics cards?
If you look at the sata power connector you will see +3 +5 and +12V:


Now compared to the PCI Express power connector:


You will see that you are missing +3 and +5 Volt. It will depend on the exact type of hard drive used if that can function with only the +12V rail supplied.

Maybe some drives will. WD Red for example does not specify a load on the 5V rail.
So that might work, since the old ATX 4-pin to Sata do work, and those do not even contain 3.3V. 

This might be different for 2.5 inch drives.