Not recommending it necessarily, but for the level of mindless cursing, complaining and bitching you want, Epic's forums can provide that.
But to Hal's point, Look at the Gears of War forum here. 3 million units of GOW2 sold (when I last checked) and barely any traffic at the BuF incarnation.
If you take every U Engine game on the market, not just Epic's...geez, how many games have used it? Certainly it's in the dozens if not much more? Running tally Hal?
40-50 different forums at least? There isn't enough traffic to warrant separate forums for just the all the titles in the Unreal series