but my bad experience with risers is that I use 4pin molex psu cable powering up 3 Risers wit GPU Rx480 8GB, and ended up burning the cable connected to PSU. 
Basically you have used a 3x splitter over a single molex cable?
I also read that sata to power the riser is generally a bad idea, this is correct?
nope, the PSU cable molex already has 3 molex capable, but were not design to handle much power, imagine 300watts for 3 GPU passing by with that wire, 2 riser with GPU is fine with single cable connecting molex, I am currently using it, I do also have Sata risers, no bad experience about them but I dont use them to power up 2 risers with GPU. its only one to one.
