The problem is that they didn't revert positional matchmaking, only positional rankings. They did this because their data suggests positional matchmaking lowers queue times. However, positional matchmaking is an issue, since it tries to place your position with a player of the same skill, but other players on the team can have their rank vary by a huge variance, and it doesn't depend on your ranking. I've had whole team's average mmr be multiple divisions lower than the opposing team's. Team of bronze's vs silver's, team of silver's vs gold's, even silver's vs diamond's. I'm silver, and I was facing a plat person in my lane. We won that game I believe, but it was super stressful for me.
Last season's matchmaking was more fairer, even if the queue times were longer. I'd rather have quality games over quantity. Not everyone has time to waste on a 20-50 minute game if it was unfair from the start. It puts mental stress on the players, and ruins the whole legitimacy of skill in the games, when it's just a coinflip on who gets the higher ranked players on their team. You didn't have this issue last season, when it tried to average everyone on the team to the same mmr, instead of just your laner.