tl;dr: I think a lot of complaints about matchmaking and feeders are in fact the result of the current state of balance and not matchmaking.
So, regardless of your experience about the MM system, it seems to me that there have been an increase in the number of threads complaining about it, often highlighting trolls or feeders who stack up 4-5 deaths within the first 10 minutes. While these threads point towards "real toxicity" or matchmaking as a culprit, it seems to me that most of these cases are a natural result of the balance directions Riot has taken for the past few years. Namely, a combination of the power creep in damage output we have been experiencing since season 6 and the "buff strengths, nerf weaknesses" idea that has been adapted a few years ago.
I'll briefly sum up the results of these two balancing tools, so feel free to skip this section if you are familiar with their concepts and the effect they had on League.
The power creep in damage is quite apparent for anyone who looks back to season 5 or any point before that. With the introduction of masteries like Fervor of Battle and Thunderlord certain power-ups outside of a champion's base kit became just as powerful as certain skills are, effectively extending a champion's damage output with +1 skill or skill level at any point in the game. This trend has been continued by the keystone runes, Conqueror, Hail of Blades, Aftershock and Predator being prominent examples in either current or past patches.
Items also got a lot more streamlined to prevent players from building too many undesired stats. Banshee didn't give AP in the past, just like Guardian Angel didn't give any AD either. Guinsoo's history and current iteration could take up a whole conversation on it's own and we didn't even mention how a support item that pretty much only gives damage boost, could become meta defining for a whole season in every elo range.
The buff strengths, nerf weaknesses direction comes right from Riot's posts. As a basic concept, they want to give each champion an "in game personality" so to speak, with clear ideas of what that champion is capable of and how it can be used or stopped. This is to prevent the Season 2 releases from happening again, where each melee champion had 1 CC, 1 gap closer, 1 damage and 1 defensive skill without much variation. The idea on it's own can be very useful and it is a staple of game design.
Finally, let's get to the point.
One result of the combination of these two concepts is that counter picks are stronger than anytime in the past. If your champion is weak against your opponent in lane, then you'll need well above average experience to simply stay alive. It is not rare that farming under tower is the only option left for a player to take and that is sometimes interpreted as being afk, or the very least the player loses lane priority hard and his team has to handle the disadvantage.
This is made worse by some other factors:
First most players are micro oriented. There are quite many who climbed up to low diamond with only micro skills and now, when the only thing that could save them in a bad match is advanced wave control, they will look as if they had been boosted to their rank.
Second, quite many players main a few champions and when their mains get counter picked, they are out of luck.
In the past, a few champions like Annie used to be the exceptions, who could kill people from full hp with an all in. Now there are plenty of examples for match ups where your only chance of staying alive in an all in is if you flash out very early on. Before season 6 if you lost a trade, you usually had to miss a few CS, a wave at most, but now any lost trade can either kill you or force you to back to the base, costing 10-20 CS and possibly a tower plate or two.
In conclusion, a lot of the complaints on the boards about feeders can be explained by looking at the match ups these "feeders" had: they often got countered and didn't know how to handle that. This will often cause toxicity or negativity by the team and then we will get to see the dozens of threads each day about MM, trolls or how the IFS is not punishing the people who are causing problems.
This is also the root cause behind the feeling of lack of control a lot of people share, since there is no real way to help a struggling player who dies in 2 seconds. Before season 6, you could rush to fights to help your teammates a lot more often, since these duels could often last 10-15 seconds instead of 2-5.
In general Riot might be right when they said that the **numbers** in their statistics show a good state of the game, but if we look at the big picture, a lot of toxicity and player frustration can be traced back to the current state and direction of balance.