I would like to provide answer why some games feels unwinnable or even rigged. We all have these games sometimes. Many players think that its fault of matchmaker or even his design to keep players in certain SR by loss streaks. In a way it is matchmsker’s fault but not directly.
When matchmaker is creating match he is trying to form equal teams. Matchmaker is picking players randomly from pool of players with his goal to have team average MMR as much close a possible to enemy team MMR.
But why we somehow have so unbalanced games then?
There are 4 main reasons and i see only two as fault of matchmaker and only i indirectly. It will be point 3 and 4 btw.
1) Reason number 1
Individual skill of players might have difference based on many occasions. Player might be tired/energized, in bad/good mood, tilted/relaxed, team might get people communicating or completely silent players, maybe is player at his peak and struggle, maybe player lost a lot of SR and is lower than his skill. Maybe is player learning new hero or has bad synergy with others. There is way too much stuff affecting how well will player play. And now imagine this for all 6 players. What if you will get 2 tilted ones, one trying to learn new hero and two who never communicate and wont switch heroes, and guess what has enemy team? 5 happy players communicating and maybe one who is trying new role.
There is way too many things like that matchmaker cannot ever predict or know, matchmaker doesnt even work with predictions, his goal is to set up match with both teams having equal MMR, job done. What you will get as team mates is often game of chance.
All stuff above is affecting total chance to win, every player placed by matchmaker into game is expected to provide 100% value for his MMR, period. Things i talked above will affect that, tilted player might get his value lower to 70%, player trying new hero to even 40%. Player who just had few coaching session can provide even 140%, smurf can provide. 200%+. Your team chance to win will be result of all these % and decisions you and others make. Does that mean that you will be stuck because of it? Not really because you are only constant in your games. You just have to do more than is expected from you. More than 100% consistently and you will climb and overcome this reason. Sometimes you can even improve % of others if you suggest them to switch to better hero for situation you face, or lower it down by being toxic and blaming. I have seen games where player’s decisions or toxicity made others leave or to throw.
2) Reason number 2
Skill rating manipulators, smurfs throwers, cheaters, boosters, boosted players, even just worse players carried by better players. These players can seriously screw up your chance to win or give you even match you cannot lose. Depends on circumstances. As i explain in first point, players are expected to provide certain %, actualy 100%, but every player who somehow manipulate ranking can have huge swings with % affecting final chance greatly. Boosted silver player to masters can provide 0-15%, smurf can provide 200 or even 300%. So yeah thats second reasons for unwinnable games, how to overcome this? Same as first point, be consistent and endure bad games, if you have smurfs, try to learn from them, watch replay and learn how they punished your team. If you are not against it, you can balance games lost by smurf by grouping up with them for few games for example. Sadly matchmaker often cant know if player is boosted of smurfing, he just see MMR what is on the account at certain point. We could certainly have better mechanisms to identify performance outliers snd push them up or down faster. But there is danger in that, deranking and boosting would be easier and faster.
3) Reason number 3
Difference between SR/MMR between team mates. Some can say its fault of matchmaker, i say it is but only indirectly as its not prefered match balance for matchmaker or for blizzard. But they are working with player base they have, if there isnt enough players in pool, more games outside of most populated elos will be logicaly less balanced with different skillset of players (low gold and high plat in same game for example).
Why is skill difference issue? Well from two reasons, not only its not fun to be low gold tank and face high plat tank but there is corelation to my first reason. Imagine that you are low gold tank and you have high plat dps in team. Enemy team has low gold dps and high plat tank. Its not great, but still kind of balanced in MMR and % players are expected to provide. But if your high plat dps decide to learn new hero, his actual skill at that moment can be equal to his gold dps enemy on other side. So then you end up with you 2100 vs 2700 tank and dps 2100 vs 2200(high plat player learning new hero). Thats 500 points difference which is huge gap. If you are silver and your silver team mate is trolling it might not be end of the world, but if your high gold team mate is trolling while you are silvet, its much bigger issue as that person was expected to provide bigger value than you.
So how to overcome this issue? Bigger player base so its up to blizzard to bring more players to overwatch. Maybe smaller allowed MMR/SR gap between grouped up players, or only solo Q+ 6 stack queue might help with that.
4) Reason number 4
New accounts. Placing new accounts is designed very poorly. It was fine when game was new, maybe devs even didnt realize that people will be creating alt/smurf accounts. Its certainly not good now, mostly because new accounts have huge swings of MMR/SR where just one match can move you up or down by 100-150 points. Also its often based on luck. You can be diamond player+ having bad games end up in high silver or bronze skilled player having luck in placement ending up in gold. Generaly speaking placing higher has a lot higher chance than end up lower. Thats very bad for new players who often end up thousand of SR from theur true skill.
So how to fix this? Better and more accurate placement system. What you can do as players? Not much, try to avoid high silver-mid plat ranks 
So yeah my 4 reasons why so many games can feel as unwinnable. We dont really need some matchmaker conspiracy with reasons i presented, they are enough to create loss streaks and unwinnable games.
Feel free to discuss.