The Main Unbalance
While the game itself is interesting and filled to the brim with fun and challenging dynamics, there's one main conflict when it comes to every bad game.
The main features of farming and getting kills and/or assists is a good idea on paper and a concept a lot of champions rely on to be able to do well and dominate as they're supposed to. Neat idea to have this; champions where you work yourself up to be powerful rather than just being op af.
But then there's champions that don't rely on this and more or less rely on their squishy nature to get in and out with clever play, which goes pretty interestingly well against power champions with counter strategies vs counter strategies.
However, that's where the problem comes in.
There's only so much you can do when someone gets too MUCH power. And it doesn't matter what kind of champion, how they work, etc. If one team makes too many mistakes or someone makes too many mistakes or gets ganked at the wrong time too much, the game's already over. The team can win, but it becomes a much harder more drawn out go in, die, repeat kind-of game from there. You can't surrender until 15 minutes in, not everyone is likely to surrender, and you'd get punished for leaving a game you weren't having fun in in favor for a better start.
The worst part about all of this is that there's no good way to farm, really, to compete with the other team. You could try getting other kills, but when it gets to team fights, especially with the god level enemy around, it's hopeless. The jungle is too dangerously risky to go into with hope of not being caught and killed. And minions don't give enough stats or xp to be considered viable. Not even the super minions.
So, in summary, having an unlucky early game, bad teammate, feeder, or simply trying a new champion that you'll have to get used to, can easily lead to a bad experience in this game rather than an intended fun strategy vs strategy style of game.