They aren't useless, but they are much harder to carry a game with.
Most ADCs are late game powerhouses that are supposed to kill everything relatively easily at the cost of being squishy. Because of that, pretty much every champion in the game (except maybe support) can 100-0 an ADC.
Now I feel that some of that needs to be adjusted. Obviously assassins should be able to assassinate, that is there job after all, but there needs to be some counter play besides "stand in fountain" or "pray for peel." Not being able to QSS Zed ult or Fizz ult when I can QSS Malzahar ult/Warwick ult/Ashe ult/Skarner ult/Fiora ult/Galio ult/Lissandra ult/etc is just BS. Being 100-0 in 0.5 seconds because Rengar ulted and combo'd you is BS. Knocking Katarina out of her ult only to have her get a reset off a teammate and then jump to me and ult again is BS. Getting 100-0 in 1 second by Leblanc Q-R-E combo is BS. Having a 0/10 Akali with Gunblade 100-0 you when you're 10/0 because she has 3 dashes and you have max 2 (one being flash) is BS.
When playing an ADC, expect to be focused. You honestly have to wait till the threatening CC is wasted on someone else and then go in to do damage otherwise you risk being CC'd yourself and praying that someone saves you. Some people get that ADC's are the main damage dealers when everyone's burst is done, others don't. The team the prioritizes the best tends to win.
I had 3 games in a row where both my support and the enemy support would CC the Thresh or Blitzcrank after their ADC was pulled. At that point, Thresh/Blitzcrank are basically useless for 10 seconds and it's the ADC's turn to do damage to the target trying to recover from the pull, so CCing them is worthless. If my support got pulled, I'd immediately focus the enemy ADC cause their initial focus will be the pulled target, giving me a few free autos before the enemy ADC may notice.
As an ADC, regardless of how behind you are, you are still a threat simply because you do consistent damage. You aren't gated by cooldowns as much as other enemies are. Obviously your abilities are still powerful parts of your kit, but your autos are just as if not more important some times. I've had so many games as support watching my Caitlyn use ult on the low enemy ADC as the Jax runs at their face where I'm literally screaming at my screen for them to just auto instead of channeling an ability that locks them in place. Miss Fortune is the one exception due to how its AOE and powerful, but if she positions poorly, it's just as bad.
So, they aren't useless, but they are dependent on either their team or on their ability to position and focus.
or Darius 
top lane. Dead. Fed zed
or yasuo
mid lane. Dead. Fed rengar 
or nidalee
jungle. Dead. I under stand that adc is the most team dependent role as you need peel (not so much in my elo but still) and protection but if you go 5/0 as jhin
in bot lane you should be able to carry. you should be able to kill the 3/1 zed or the 2/0 kha zix. Not sure if im just missing the jist of adc and this is how its always been but its been getting really bad recently. im not saying these assasian champs or brusier champs are overpowered (also not saying they arent) just dont get why if i do better in lane im unable to beat them out of lane.
