Hi everyone!
This has been my personal experience. I am someone who has been in Diamond, Plat, Gold and even low Silver. Low Silver was easy wins and I had four golds and 70% kill participation. Gold was similar. When I hit Plat then everything changed. I was at nearly 30 more wins than losses on record. My Soldier stats was top 97% on Overbuff, same with DVA, Zen and Orrissa. But when I hit Plat the enemies were more challenging and I was NOT able to get four gold in every match anymore and I was no longer getting 70% kill participation. I actually started losing but my record is still nearly 30 more wins than losses.
In contrast, when I was in Diamond I mostly lost matches and kept losing until I dropped down to Gold and then climbed back to mid-plat and stuck there. I think this is mostly my skill cap to be honest and not any rigging of the matchmaker.
Below is my theory and personal experience with the matchmaker.
If you perform extremely well and carry super hard and can do this consistently then the system will put you in fairly easy to win matches where you do not need to carry because you have a great team who all have great teamwork and focus the objective together.
If you perform super good but die a lot even if its because you have no good healers or bad tanks then it does not matter how good your accuracy, elims, or damage was. All the system focuses on is your K/D ratio. If you died a lot then the system figures it was your fault the team lost even if you had 75% kill participation. So in this case having four gold is not good.
So when your climbing the system does not see you as a threat causing your team to lose. If you do lose then you lose very little SR and if you do win then you gain more SR than usual cause your climbing and consistently performing good and not dying a lot and are frequently onfire throughout the match. If you keep this up then you will climb not only due to your performance but also do to the system putting you in easier to win matches.
However, if you get into a match where your team is destroyed and you die a lot (even due to a leaver), then the system thinks that you may be too high now and you caused your team to lose by under performing or dying too much or you did not go onfire but your teammates did.
If you end up having a few matches where your losing over and over again and now suddenly find that the matches seem impossible to win then this means either the system is putting you on hard to win matches to help you go down a little cause you causing your team to lose (what the system thinks even if its wrong) or you climbed too high and you really are struggling and are not carrying as much, dying more often, and not onfire as much cause your playing against higher SR/MMR enemy teams and you are at your limit as far as skill goes or a combination of these.
However, if you suddenly were able to start performing well, getting onfire a lot, not dying hardly at all, and winning one sided wins rather than close calls that you barely won then the system will start helping you climb again.
That is what my personal experience has been with matchmaking.