i think the reason why matchmaking sucks is because it doesn't account for inconsistent performance
if you are the type of players who play like a god one game then like a person who somehow skipped the unskippable tutorial an other game then matchmaking HATES you because it doesn't care for inconsistency
if you did well the last few games and now you can't perform? too bad ur mmr was buffed from those games and you are expected to always perform the same or the game will be bad for your team (usually)
if this doesn't apply to you it will certainly apply to at least one of your teammates most of the time