Blizzard's Jeff Kaplan made a post to the Battle.net forums with some "personal" thoughts on matchmaking, though of course him being the game director for Overwatch makes his opinions on the topic significant. He discusses the goals of matchmaking in Overwatch, what the system takes into account, and how they try to calculate matchmaking ratings. He goes into detail about MMRs, and clarifies that there is no specific effort to force a 50% win rate. He also discusses how winning and losing in different manners impacts players' perceptions of how well matchmaking is working, some of which reflects upon human nature more than computer algorithms. He concludes with an overview of the situation preceded by an amusing anecdote about how they realized their "avoid" system was a problem:

We are constantly improving the matchmaker. We learn more each day. We have one of our best engineers and best designers full time dedicated to the system. Many of those “silent” patches that go out during the week are adjustments to the system. For example, we recently realized that “Avoid this player” was wreaking havoc on matchmaking. One of the best Widowmaker players in the world complained to us about long queue times. We looked into it and found that hundreds of other players had avoided him (he’s a nice guy – they avoided him because they did not want to play against him, not because of misbehavior). The end result was that it took him an extremely long time to find a match. The worst part was, by the time he finally got a match, he had been waiting so long that the system had “opened up” to lower skill players. Now one of the best Widowmaker players was facing off against players at a lower skill level. As a result, we’ve disabled the Avoid system (the UI will go away in an upcoming patch). The system was designed with the best intent. But the results were pretty disastrous.

We will always be working on our matchmaking system. We’re listening to feedback, we’re playing the game a ton ourselves and we’re looking at hard data to inform our decisions. This post wasn’t my way of saying everything is fine. I just wanted to share some of my thoughts as someone who has been evaluating the system itself very closely as well as monitoring the feedback. I want to put it out there that there is a lot of room for improvement but also suggest that there are forces in play that cause some fair matches to sway lopsided due to forces out of our control. The game is as much (if not more) art than it is science. We’ll keep working to make it better!