I noticed with harder UQ, plenty of players just decided to leave if even 1 little thing goes wrong (if that). I been noticing this a lot more for the Dragon UQ. If they go down they rather leave than be picked up (obviously this effects your grade.) but that isn't the rest of the parties fault. When you want to report them from the recent players, you can only blacklist them as they either left the server, block, etc and you can't report them if they get off by the time you are finished with the quest. The most you can do at the moment they leave is blacklist them. We should be able to report them once they leave to stop them from doing that.
Edit: One of the reasons I normally play with clan however, this still is a very bad habit of the player community that shouldn't be ignored. I remember on Saturday I had to finish the Dragon with my sister because the other 2 players left. (However 1 was disconnected.)