I personally prefer PCIE 6pin type (riser version 6?) so that I can reuse my 6pin PCIE cables especially when using server PSU.
I have many SATA powered risers in production now and all ok so far.
I guess its a matter of good quality build instead of "dangerous", and if your SATA cable connects via molex onto the riser -- ensure that its tight and secured. I suspect that the real danger is when low quality molex doesn't get in proper contact or loosely contact with the 4 pins -- thats where trouble starts... ie. sparks, fire hazard, damaged riser eventually over time.
For risers I usually take good look on the quality of the USB3 cable... the ones that are "fat" or shielded likely have 100% uptime compared to the flimsy ... phone charger looking type USB cables...
so with PCIE 6 pin risers, you go 6 pin to 6 pin cable directly without using any adaptor?
its like a Y cable (one for GPU and one for riser) .... one single cable that goes back to server PSU
look closely here.... one cable splits into 2 pci connector, an 8pin for GPU and 6pin for riser... this way power goes to both devices at the same time so consistency perhaps makes a stable rig....
who sells those Y cables you talk about.. also how long are those usb cables your using?