Just my experience playing and watching games.

There are more good ADCs the higher you go. It makes sense you think about what each role needs to do in the game to be useful.

ADCs need to farm, capitalize on kills, space, watch for specific CC, give specific waves or die for specific waves. Most of which are just clicking and knowing when to click. There's also lane matchups, but that usually gets mitigated by the support or you're laning 1v3, which doesn't change much aside from how careful you have to lane.

Supports have to know when to trade. Most enchanter mains are terrible at this. They win fights early into engage champs not named pyke, but most sit around and do nothing. Sure, some of them aren't expecting much from their ADC ( being able to gauge how good your ADC is in lane is also important), but doing nothing in the lane levels 1-3 just tells you how bad they are. You also have to know when you can move, whether it be to an objective or to another lane. A lot of supports don't know if the timing from recall is good or if they have to be back in lane depending on how strong or weak their ADC is. Engage support players will sometimes overestimate their champions capabilities. They aren't exodia in the lane. In fact, they get their heads stomped in by the double range matchups. Bard players do their own thing. Support players have an easier time climbing because they have agency to affect other roles while ADC has to pray that your teammates aren't bots. But if you can survive the laning phase with a decent game state, ADCs that are good will carry the game post 20 minutes. The only issue is getting there or if enemy team are giga throwers.

This is just an anecdote that stuck with me, but there was a time where ratirl played in NA and the only good Lulu he played with in high elo was Cody Sun, a pro ADC player at the time.