Meh, people complain about the baddies that they get, and it is a relevant complaint, but not to the scale as some people make it out to be. There some flat out bad players who for some odd reason get into your match and pretty much are worse then dead weight in every game they play, there are the people who are just having a bad/very bad day, sometimes you can't help losing lane hard, there are so many variables as to why a person who is averagely quite good isn't as good in a match, stretching from things happening in game, to real life as well. Then there are trolls, people who could be bad or good, but intentionally play bad for the sake of enjoyment of the suffering of their team or just because they got mad and are too immature to put their rage aside and play (then again this is probably a lot of the community, just take out the trolling part). Yes the matchmaking seems to mess up every once and awhile, but overall, I think its more equal level players (or accounts ) that are having a bad game/day that are the issue. That being said, there are some blatant issues with the system, for example, while i was a 1450 ELO Player, and the other 3 members of our team were below me, we randomly got a 1600-1700 ELO player on our team, and the enemy team consisted of above our average players, so I assume they were higher ELO then us as well. A 200 ELO difference is pretty huge, and that kind of problem can get very irritating to both parties. [The people who are getting outplayed by better players, and the people(person) who has the team of noobs (by his standard)]
TL;DR People having bad day are more of the issue, then the issue is over-dramatized by mad players in the community, but there are still issues in the system.