Aphelios' win rate increases by 8 to 10% after 10 games played and plateaus at 60% within 20 games.
His mastery curve is around 10% or more, one of the highest mastery curves(the highest being 12.7%), but players only need 10 games to master him and even after 50+ games they barely see a difference because Aphelios is just another AA reliant ADC, the only difference is he rotates through his abilities.
I think making him an ADC limited what Riot could do with his kit. An ADC can't have too much CC and utility cause it'll be problematic, so he should've just been a mid lane mage or an assassin because then they could've made him into another Lee Sin or Azir. I also think not giving Aphelios the choice of what guns he gets to use and not making the guns distinct and unique enough ends up defeating the purpose of making a champion like Aphelios to begin with.
edit: source for the stats is leagueofgraphs