The reason mm is not the best should be pretty obvious. Taking into account the playerbase, 80% (generous) being low plat (in skill level) and below between both console and pc. The other 20% ranging from avg to the pro level. Many of that 20% using smurf accounts to help friends in ranked or just pubstomp. The option to include or disclude console or pc players from the pool of which the mm takes from. Factors such as account lvl and w/l ratio are suspectable enough to be part of the mm algorithm but I don't know if anyone besides the devs know for sure.
I would think if mm largely was based on total hours played, avg kda among the previously mentioned factors and considered the majority in queue with the closest related stats among those variables, that would be the avg it would try to create a game from. Perhaps 7 of them range from level 70-150 and another was brand new another level 300 and the last a level 500, and no one else is in queue to pull from itd have to make do. Those edge cases for high level players exist and it seems to be how the mm is. So considering this, if 80% of playerbase is below avg and a similar pool from that ratio is in queue, it wouldn't be farfetched to gather that 80% of your games may revolve around that level of play, mixed with some newer and some better players. Does this sound any different than any players currently experience?