Here is a great and detailed post about how MMR negatively impacts Competitive. Algorithmic Handicapping (MMR) is Wrong for Overwatch
It’s long, so I’ll summarize it.
-MMR is the game’s hidden judgement of how good you are, relative to your SR. If your MMR is high, then you have been judged to be better than the average person at your rank.
-The matchmaker will put you into games where each team’s average SR is the same (and not to spread out), but it also takes into account MMR, and it tries to make your opposing team as good as your team on average.
-This means that if you’re good for your rank (and deserve to rank up), your MMR is high, and you will be matched with teammates with low MMRs to balance it out, so each team is equal.
Basically, if you deserve to rank up, you will be deliberately matched with worse players to make each team equally likely to win. This helps worse/new players and hinders better/experienced players.
It essentially enforces a 50% winrate. This is what is referred to when players talk about “handicapping” in Overwatch.
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This also increases queue times. By a lot.
Let’s say you queue for DPS in Platinum ("< 6-minute queue"), and it takes 10 minutes. There are tons of people playing at your rank to match with, but you aren’t being matched with or against them because in role queue, you need your tank players, support players, and damage players to equal the same MMR as those on the enemy team.
This makes the shortage of tank/support players much worse.
There could be some sort of bug with matchmaking that’s causing these long queue times in part, but MMR has always been a contributor to long queue times, along with many of the problems in Competitive, including toxicity, throwing, smurfing, and general inaccurate rank.