Match making has some serious problems this year, albeit Riot refuses to admit it. I think a good portion of it are pre-made duo's breaking the system. Basically players duo'ing at extremes, like Plat I with a Gold IV player, causes massive problems. Basically the enemy team will have like all Gold II / III's to counter balance against the enemy duo, but realistically a Plat I still destroys players at Gold II / III range causing a lopsided match.
The current META doesn't help either as the hyper carry meta is back with champions such as Zed being able to 100-0 almost anything in the game regardless of their build. If you aren't playing an S or S+ tier hyper carry then basically you are relying on your team to win, which usually is like rolling a pair of dice to see if you're going to win or not. It's like basically, if you and your teammates are not playing one of these champions: https://u.gg/ then your win potential goes down astronomically. Meanwhile if the other team has more of these champions, then their chances of winning are much higher.
In the past the difference between a God tier and A tier champion was actually very slim. The "outliers" of the game were the low points, meaning those who fell into the C range. Most of the time up until Season 7, a B tier champion could still carry against a God tier champion if they could secure a lead. This was mostly due to how efficient Armor and Magic Resist used to be (most 'tank' items had more resistances than currently).
Starting around Season 7 up until now, the exact opposite is true. Since the end of Season 6 / beginning of Season 7, nearly every tank item has been gutted (Banshees turned AP, GA turned AD, Resistances lowered across the board, health lowered across the board). This in combination with new Keystones has created a META where damage and penetration prevails against resistance stacking.
The effect this has had on the game is that it made God Tier champions supreme (new outliers, unable to be matched) with A / B tier champions having no fall backs to help push them through laning phase. Previously where in bad match ups you could grab a few resistances to stall lane (or some damage, like a Tiamat in the case of Tanks like Shen), now it only puts you farther behind with no advantages to gain. The enemy still hits just as hard whether you purchased the resistances or not, so basically the only option is to go with your original build without variances. Increasing the overall damage in the game has led to less build adaptability, and in turn leads to more 'snowballing' game play patterns.
Basically from A tier champion to God tier champion is basically the equivalent of D tier to A tier now and really low tier champions tend to be complete garbage regardless of how well they are played. In the past the difference was a lot less between high tier champions and even lower tier champions could squeak by in most unfavorable match ups.