All we really know is that risk increases throughout the synthesis and decreases the chances of success while increasing the chance of instability. However just like bad element colors, high risk increases quality if you are successful. That's why you get 2/3 of your quality in the last half of your synth if everything goes well.

I use high return 3-5 synths from the end of a session just because, but I can't honestly say I can tell a difference without stat software telling me there's one.