It's always been broken. Every matchmaking system is. In fact, it's not even possible to have a truly balanced matchmaking system when one has to account for so many variables.
It's exacerbated even more because of the inherent snowball nature of the current game. It's almost impossible to come back from a slight gold or objective deficit now.
I'm paraphrasing, but last time I looked, first turret, blood, or dragon, not mutually exclusive, lead to a 65%-75% game win chance. I'm not saying these factors are the cause, it's possible that different circumstances lead to one team getting the first blood, tower, or dragon, but it goes to show how quickly the game becomes statistically difficult to win.
But just knowing this makes it even worse, anecdotally, as I (and I'm clearly not alone in this thought) immediately know how difficult the game is to win after falling behind early, which makes the team even less enthusiastic in giving their best effort.
Next thing you know, FF spam starts at 15 minutes. The team takes more and more careless risks, 0/11 full AP Amumu top starts running it down mid and everyone comes to the Boards to complain.
I don't think matchmaking is the blame for all the problems, as there's always going to be five losers, but it, when paired with the many other problems, certainly doesn't help.
They need to simplify the system and do away with the ELO system completely. No more hidden values when queuing, but queuing based on visible rank. I don't want to be Silver 4 playing against a Gold 1 in lane because an imaginary number says we're equally skilled. Queue us against our opponent based on our visible rank and if I am better, I should win, and move up. Eventually we would be in the rank we deserve, queuing against the players who are our equals.
This average of team MMR for matchmaking is stupid. A 1500 MMR team could be five gold players, or an iron, a bronze, a silver, and two plat players. (Numbers made up for effect, but you get the idea).