It's not just adc's that are a problem but botlane in general is horrible.
Have a botlane get ahead early? Snowball and basically coast on a free win. Have a botlane feed their asses off? Pray to god the enemy teams adc is stupid enough to let you get near them.
True, this does apply to the other lanes but having a fed adc is basically crucial to winning teamfights and pushing as a whole team down a tower. Yes, their are split pushers like tryn and nasus but these champions usually rely on having a great laning phase and snowballing that momentum into the mid to late game.
You can have a top laner go even and still contribute but if you're carry lanes (botlane) don't do great it's basically a death sentence just waiting to happen.
I'm just honestly done having to hope that the botlane i get matched with does great but really that's like asking for a miracle to happen in general.