I have been reasonably content with the success rate. It hasn't been successful every time and there have been times where the response time was wanting, but generally not too bad.
I think the last unsuccessful request was about a month ago in the Lost Thread. Something happened with Ana and one of the objectives didn't check off. I completed rest of quest, but didn't get xp/credit and GM couldn't help. But that has been the exception rather than the rule.
However, I did just submit my first ever ticket for an item missing from my inventory and I am still in a holding pattern regarding whether or not it will be restored.