That's because they are a bit too good at doing a bit too many things.
This is coming from very personal experience as i'm a support main,but hear me out:
Adc's can take down tanks quite easily,as well as burst down squishies when fed,and even surviving burst champs because of extremely high life steal and burst because of their linear build path (usually 4-5 crit/atk speed items->life steal->anti tank item)
They're also really good at pushing and split pushing,meaning they can apply a lot of pressure on the enemy team,forcing them to take out the adc before they reach inhib,since they're amazing at destroying towers.
Then they're also amazing and even often necessary when taking neutral objectives such a drake or baron.
They're also a really good source of damage.
The reason alot of people hate them is mostly because,even if you have a fed assassin,a tank can focus them down and build against it,same with mage, or assassin->mage. However,to stop a fed adc,you're gonna have to usually throw your entire team on them as tanks won't work,assassins will often get burst down if they're fed enough,etc etc.
The major problem is that the team with the fed botlane usually has a really great chance of winning right now,so bot is high risk high reward in a way.Fun and good for the botlaners, really shit for the rest of the team if the enemy botlaner happens to get fed.
