While I definitely hate the "it's just [insert current game mode here]" people, I just can't take flex seriously at all. Not that I wouldn't try to win if I still played it, but the queue itself is MMR Russian Roulette. I have played against and with anything from Bronze V to Master Tier in Flex queue. If you thought games were one sided in normals or solo-queue, then you haven't had the joy of playing a few Flex queue games.
Just as an example - My team once consisted of two Diamond 1 players (one of which was Master tier solo queue the other has been Diamond three straight seasons), a Plat III (with an 80% win rate in 50something games), a Bronze I and me in my provisionals, while the enemy team had a Plat II support main, a Gold IV, two Silvers and an unranked. I've played thousands of League games through the years, but this had to be top five most one-sided matches I'd ever been a part of... I was mid and went something like 1/0/10... basically, as you would imagine, the Diamond players had their way with everyone and the game was a 16-minute stomp, one that we likely could have won even faster but we decided to take Drag and Herald.
Granted, this was shortly after the beginning of this season when MMRs are haywire, but come on.
I have also been on the business end of these lopsided affairs. The point is... for those that say ranked is about "luck", I would usually disagree, but Flex queue is definitely more luck based or, at the very least, centered around who you know and queue up with.
It is absolutely a joke.