Stats seem to show that USF is the strongest faction.
I can't believe how many people reference that post for balance arguments. In the post Siphon literally says don't use for balance arguments. I mean most of that data comes from before the last patch...
Even if you could use it, it shows a whopping 4% difference in winrate, and that's for all rankings. The graph that breaks it up by ladder position shows the winrates fluxuating all over the place