I also found it strange that even at lvl 70 with melded i290 gear i was getting frequent failures quicksynthing 68 recipes, and on occasion lvl 66 too. I looked around and saw people with much higher stats than mine still reporting failures, the highest was over 1500 craft and still failing on lvl 68 quicksynth.

I have no idea what is needed to get 100% success on these but I have a feeling that SE purposely made it either extremely difficult or impossible to do during this patch as a way to prolong the grind for Gyr Abanian Alchemic. For instance I just turned in 30 Gazelleskin Brais of Scouting which required crafting 120 twinthread and making 30 twinsilk from those, plus 60 gazelle leather. All of these are lvl68 and I had to manually craft them all because I didn't want failures from quicksynth. And after all that I still didn't get enough srips to get even half of the achemics needed to make all of the mainhand tools.

So basically, instead of dealing with weekly capped scrips we get to choose between manually crafting thousands of mats or loosing a bunch of mats to quicksynth.