The problem with UGC (User Generated Content) is, as anyone dealing with UGC already knows, err..well, all those Users really. They make a lot of noise—some is interesting, but most is noise. UGC can fill your site with a lot of content, but how much of it is 1) relevant 2) monetizable?
Example: UGC problems at YouTube. While they are serving 1.2 billion streams per day how much of it is worth watching or not infringing? YouTube has a commercial partner program to try to sift out the good from the bad and reward stuff that adds value. It is hard to imagine just how many take-down notices they process each day? Millions? When YouTube kills an account for breaking the community standards, there is a flare of angst against them.