Missions are the worst thing Riot ever added to the game
Nevermind OP champs and busted items, forget unbalanced runes or jungle changes, missions are the worst thing that's ever been added. The psychological impact they have on the game absolutely ruins it whenever there's some new terrible event going on.
Don't feel like playing right now? But there's limited time only rewards! You want rewards, don't you? Better login and play anyways even though you're going to just do terribly since you have no actual desire to play at the moment.
Hey look, enemy base is open, go for the win! But wait, you still need 2 more barons to complete a mission even though the time you'll waste getting it would be better spent ending the game, but who cares about playing properly or winning when you have unnecessary side objectives to accomplish?
Terrible at ADC? It's your worst role? Hate that new game mode that's all ADC all the time? Well too bad because you have to play it if you want to complete all the missions.
Game time just hit 33 minutes? Time to surrender. But we're winning? Who cares, surrender anyways. I'm not putting forth another ounce of effort from this point on, why would I? There's zero incentive to keep playing win or lose, because every second longer the game goes on is just a punishment. And that's assuming we're winning. Assuming we're losing it's even worse. A team that won't surrender an unwinnable situation is bad enough when you're playing normally, but it's even worse when missions are involved and there's a ticking clock. The missions themselves encourage you to feed to end the game faster so you can get another shot.
Missions are just overall terrible for the game. They warp normal gameplay incentives, encourage you to play sub-optimally, and just overall make playing League a miserable experience.