The clowns usually never lose, at least not before the very end. I don't think it's just them though, because among them we know only Uta to be strong. Not that we have seen any of them fight.
Aogiri Tree on the other hand is overpowered as we know. Tatara, Eto, Sachi, Noro are very strong and there could be more.
I think Uta is actually stronger than Yomo but that's just hype since Ishida has never shown him fight seriously.
Why the hate for clowns though people? The clowns did nothing wrong, they just enjoy creating a mess it seems. Also they didn't betray Kaneki or anything, since neither Uta nor Itori said they were his allies or friends or anything. Uta and Itori were never with Anteiku, they were always at a distance.
Aogiri tree seems to have a goal which we don't understand for now ("to smoke out a liar"...dahell?) whereas the clowns just observe and manipulate the events apparently.
P.S. Don't forget that there's another group too, "V'.