The official Matchmaking blog post

Yea, it all works out in the devs brains and their artificial sample data.
It doesnt account for Hot Drop Harry who pulls down everyones MMR in the team, it doesnt account for the suboptimal hitreg and bad servers here in europe ...
In that case you play focused on placement ... and again get  punished by the system
Your MMR will get lower and lower by being matched with brainiacs that got their rank and badges out of a vending machine, no wonder they lost over 3 million players in ranked since they introduced their matchmaking changes in S20 ... its simply not tailored to fit the needs, but for a certain target group ... which also now leaves the game in masses ... bet on the wrong horse and loose everything in the process.

The MM was focused on triple stacks and pro streamers needs for far too long , but most people dont have a set team nor are they the beloved content creators.
As i said , it might work out in theory but the reality shows ... people are fed up with all things related matchmaking  ... respawn really has a way doing things worse than before with every iteration.
The devs write about the MM being a complicated system ... what happened to K.I.S.S. i would ask ? A major design principle in software development being thrown out of the window.
Complicated  systems are prone to bugs/errors and can hardly be maintained by others that didnt take part in the initial build process, which leads to even more errors.
Building on top of a flawed system is like building a brickwall house on quicksand, the house itself might look stable but that doesnt matter if the soil it is build on is fragile.
Patch after patch after patch ... sometimes its just better to let go and build something from scratch.
And i strongly believe that quite some engineers at Respawn told exactly that to their higherups , but their ideas got "killed" by the "upper foodchain" for the well known short lived business reasons.

This is your excellent matchmaking. After a whole day of failing and dying quickly, Wattson suddenly appears in my lobby again and has 20k kills as the kill leader. Her skills are certainly as good as mine. What is this even supposed to be? This game is over.

2 years after the original blog post about matchmaking and here we are in the exact same place.  Player counts droppipng like a rock and absolutely nothing has changed.  This should be taught in college computer science curricula . . . "How to Create a Popular Video Game Service and then Completely Destroy It 101".