if a tank fully engages on you and you cant take him down with your team, it means you are allready very behind. its like if an enemy azir is super fed, he oneshots your entire team from 5 screens away, but thats how it is when things get out of control in league.
if you are all even and you cant kill him, either your champions suck or your build sucks or even your runes and masteries suck. if you picked, built and set up properly, tanks are no problem.
sure at 1 item (IE vs randuins) a tank is a very real threat to an adc and he definitely cannot duel him, but allready at 2 items (if the tank bought another tank item and you bought botrk/PD) he will allready be no problem and this just continues. also, in most games you cant just build one sort of resistance, and for every MR item a tank builds, he has 1 less item vs every ad champ, so basically a full build maokai or singed will still melt against a full build jinx or vayne because you have 6 damage items while he doesnt have 6 items against your damage type.
also, cho and singed are exactly like maokai in the sense that they dont all-in walk into the enemy team but they rather walk up to them, fuck around and run away to waste their time and use their cooldowns again. sejuani and amumu work a bit different because they have no inbuilt tankyness, so they cant really fuck around too much without taking substantial damage, even when they build full tank. also, most of their abilities require them to go into the enemy team fully, so thats what they want to do and then instantly win the fight.
tho their weakness is if you can ignore them. and by "can", i literally mean "can" in a mental sense. if your personality is strong enough to not attack an amumu who just ulted 3 of your teammembers but the yasuo who charges in right behind him, amumu is not as usefull. he can ONLY all in you, but he cannot waste your time as effectively.
sejuani somewhat can do both, all while dealing alot of damage, but since the nerf, it isnt that much anymore so she really became a timewaste-bot without too much capability.
right now, gragas is the way to go if you want to be a real asshole. he is UBER tanky because of his drink, he has mobility, knockbacks, aoe-movespeed slows and a giant teamdisrupting ultimate, all of which deals massive damage and regenerates his health.
he is the embodyment (haha) of a cancer champ. literally no purpose in life than to fuck up your enemies with maximum gayness. walk up to them, drink, slow them, walk away, if they chase, bodyslam into them, walk away, scatter them with ult and possibly ult one of them into your team, bodyslam, slow them, bodyslam, walk away, drink, bodyslam.
no matter how behind he is, he is always usefull in some way as his abilities all have certain impact by themselves.
he is also the prime example to show why tanks are tanks. they are not tanks because they buy defense items, they are tanks because they have a kit that gives them survivability all while allowing them to waste as much enemy time as possible.
is great on paper it really really is. Especially on champs like Twitch, Vayne and Varus. But 
Just feels so weak. And most tanks will have enough armor/mr to shrug these 2 items off like they're nothing. I'm mainly just frustrated at seeing 3 tanks every game, tank support, tank/bruiser top and tank jungle, that's not only boring as hell but overall just a complete pain in the ass to deal with.
Who pose a very real and high threat to everyone because of their tankiness. I'm not saying that tanks should be useless, quite the opposite really, I think they're great right now and in a very good position. But when you have 3 tanks on the enemy team that are capable of facetanking you while dishing equal, if not more damage back at you. Then at that point I feel like there needs to be some alternative to fight back against it, while staying in check to maintain balance.