I just hit S rank, but my experiences here are based on playing at A+What rank are you at? Could be these are special circumstances since I'm stuck at A+ with the S/S+ in the same pool situation. But Open has matchmaking problems in the opposite direction as I've been in games where I was fully convinced, while playing and after reviewing the replays, that everyone else was C+ at best.
I also play Open a lot more so my anecdote might not mean anything for Series. But Open is the only one you can play with friends so the chance of running into a coordinated team is much higher. I also ran into a professional group that was streaming on Twitch while doing Open. Needless to say, we lost that one lol