It depends on the adc you're playing. Some are able to perform well and be carried by a team without much effort.
For example:
really don't require much skill.
Cait: You just need to learn that your game is a defensive one. Get people to chase you, set traps, and turn their chase into your kill. Long range, relatively safe csing, and hits farming stride fast.
Jinx: extremely strong wave clear on first item, long range, and can be build with many different rune combinations. Her Ult if you can learn to land it, will minimum give her 10 kills a game, even if you're a horrible laner. And if she gets her first two kills early, once she has a
, just owns the game as long as you know to stay behind some damage peel.
Sivir: greatest Wave clear in game with Q and W. Gets to ~ 50% Crit and she just can farm 2 lanes at once, and get into team fights in between. Her E is usually enough shield to turn most fights around.
Medium skill adcs:
: Primary problem is learning to auto attack and abuse his passive after Q's.
: Primary problem is learning to aim Q's. If you can Bounce Qs and always hit a champion with the second hit, you almost always win. The output is gigantic.
: Seems easy to play to people who don't play her, but her early game range has been scaled back, and her primary issue is resetting her W and only diving when she can ensure kills. Has good clear, awesome damage, but often gets tricked into feeding with
summoners. Once you learn her, she's monstrous. Before you learn her, you're a liability.
: Harder to play correctly than Twitch mains make him out to be. He's relatively low range off ult, so gets bullied often in lane. Needs to learn to use W in pressure lanes to toxic minions and farm them without auto attacking. Needs to also learn not to get isolated or cut off from an escape path if he wants to reset Camouflage. Needs to learn not to start fights, but end them, because of how his ult works.
Decent damage long range skill, stacking on hit debuff that triggers on skill hits. Mechanically slightly above average difficulty, but carried heavily by 
Getting used to and using her Evolved skill versions takes time and effort. Most use them wrong and only focus on using Ult to engage a low target, and Q them. That's improperly using champ. Gets semi bullied in lane. She's freelo vs someone isolated because of her Q. Minions screw her.
Hard skill adcs:
Timing her Rends, and link synergies, and using her hops correctly takes time and effort to master. If she fails and puts Rend on cooldown, she's a dead adc. Has 1v5 potential, but a highs kill cap.
Horrible if her move speed buff goes on cooldown. Laying feathers correctly takes talent to do without a team. MUST get fed early, or will get shut down most of the game. Rewarding if mastered.
Contrary to the Ire she creates. Pretty hard champ to master. Low range. Most of her damage lays in being able to use Tumble correctly. Her kit is entirely about all in, so if she's forced to run, she's going to die. Also, her Tumble is annoying because you can't precharge it and put it on cooldown. If you use it, and don't have a target near you to attack, you will create about a 6 second window where you cannot reset it. You will get assassinated in that time. Has no innate aoe wave clear. Needs to master csing.
Needs to get into auto attack range to be dangerous with skill resets. High potential, but also has a feed potential.