I could write an essay on this (played since 2015) so I'm sorry if this is overly wordy and I hope it makes sense.
A long time ago they had a pretty basic MMR/matchmaking system that was being gamed pretty badly by smurfs. So, in an attempt to deal with smurfs, Epic made a really opaque blackbox "internal" MMR, that determines what kind of teammates you get. The reason this exists is because everyone tends to get lumped into "chokepoint" ranks - Diamond 3/low champ, C3/GC, and from there I think there's another one closer to SSL but I've only been a GC1 before so I can't comment on that experience. The reason for this is you can have a 60+% winrate in diamond and never leave diamond. That results in tons of players being in a rank they clearly do not belong in - very ironically creating an unintentional smurf problem they were trying to addresss in the first place - and this is exacerbated by a draconian rank decay/reset every season which only lasts a quarter. So, it's not entirely uncommon to see players with a GC tournament tag from the previous season (means they won a GC tournament, which means they almost certainly are SSL players) in low champ or even diamond. This is made even WORSE by the way that placement games work every season - pros have worked out it's almost always better to just sit out the first few weeks of a season before doing your placements, because you may accidentally get a really high "internal" mmr (the only purpose of these placements) by beating other smurfs in a low rank ladder.
As a result, epic has tried to balance this idiotic mess by pairing these types of players with lower "true" ranks. So, often when I am in high diamond/low champ I will have a literal carried platinum player with me every single time against another team with a guy who also has a higher "true" MMR and whatever "low" MMR player the matchmaking has decided he also gets paired with. I solo queue though, so this effect is basically a miserable experience trying to climb, as it's much more difficult for me to climb out of a low rank than other players closer to their "true" rank. And I have to repeat this process every 12 fucking weeks.
when I play casual, within 10 games the matchmaking immediately puts me in high champ lobbies. So, they know how to match make fairly, clearly - there's simply no mechanism in ranked to quickly rank someone up who very obviously should not be in the rank. It should not be possible to win 15 games in a row and stay in the same rank - every other game addresses this by quickly ranking up people on win streaks. For whatever reason, epic does not, and for this reason and all of what I listed above, the experience is absolutely miserable.