I figure I'm really late at answering this (I'm predicting Fillychan going on hiatus inside of a week).
I'm probably a very inaccurate observer, but I remember United Chans as an attempt in 2013 at creating co-operation between a number of pony-related imageboards. Ponychan joined in the summer of 2013, and promptly forgot about the UC's existence less than half a year later, unless there's been some internal cross-site mod discussions that I've never been told about.
My chief memory from it was that United Chans gave its support to one guy who was to set up an interchan PES tournament (It was originally meant to be Football Manager, but /mlp/ complained about it). After Ponychan, with me among the three main team contributors, finished making a team squad (though the info on the squad list was very lackluster by 4chan Cup standards), the tournament planner began focusing on his studies and then disappeared. That wasn't only the first time I've been cheated out of interchan cup plans, but the second.