i did a lot a quick synth with very good stuff (2150 craftsmanship, 2200 control, 520 CP unbuffed) and it seem that the failure rate is a bit higher than in Stormblood 4.0.
I did not notice any real difference between my yellow scripts gear and my good gear, and 85% success seems about right (after maybe 2000 quicksynth).
It has always been a mystery to me about what attributes contribute to decreasing failure.