On one account, I had some extra DBG cash so I decided to "Finish Now" some quests. I'm 0/5 on successes (all failed) on the ones I finished using DBG cash. All of them had 80% or higher Success probability. TBH, the only reason I continued to spend coins was to test if it was broken. I'd rather not test any further with my currency (am only out maybe a hundred or so coins so no biggie).
There definitely seems to be an issue with the displayed success % and actual values. Multiple people in my guild, including myself, are experiencing failure on most quests despite them being in the 84-91% range.
I failed a 91% success rate one as the first one that finished. They really need to fix the instant finish now button to use what it will really charge you. It seems it is based on the finish time left but because it is calculating everything off your computer's time it could be wrong and I got charged 52 instead of 40 for instant completing one during a test.