For instance, every other champ in the game has skills that scale with level. You have to choose what skill to max first, and hitting certain levels (6,9,11,13,etc) for that spike in skill based damage is a huge portion of this game.
This matters to ADCs less than most other champs. The reason that ADCs go bot with a support is that they dont need XP in the same way that a mage or assassin mid needs XP. ADCs scale far harder off of gold than they do off of XP, their skill damage is mainly to get them through the early game to the point where they can just auto everything to death.
Aphelios doesnt have to deal with this. He gets free stats per level instead of having to scale a skill, yet all of his weapons and their Qs do good damage or have great utility from the start. IMO if other ADCs could level stats instead of skills and instead have a higher scaling on their skills, most would go for it. Aphelios also doesnt have a niche. His niche just depends on whatever weapons he has atm. The problem with this is Aphelios is the best at whatever niche he has when he has said weapon, yet can then go be the best at another niche when he gets a new weapon.
What do I mean by this? Take Aphelios's starting two weapons - The sniper and the pistol. With sniper, he has Caitlyn range (650) so he can easily harass in lane, or farm from extreme range if he needs to play safe. Caitlyn used to be played A LOT because of how much of a lane bully she was, but now she loses to Senna and Aphelios. because Aphelios, while he has the sniper, does what Caitlyn does, but better. If you engage on caitlyn though, she might have to back and leave lane; Aphelios gets his pistol out and gets some of the best sustain in the game.
His pistol, when its up, makes Aphelios one of the best sustain champs in the game. There is no other ADC with the capability to build 0 lifesteal and yet heal from 10% to full after getting chunked out of a fight.
You could have a full champ based off of these two things alone and he would be great, but Aphelios is an infomercial.
His next two guns are gravity gun and the flamethrower. Gravity gun, when its up, makes Aphelios one of the best, if not the best, utility ADCs in the game. Ashe used to be the premier utility ADC, providing slows and really good hard engage with her ult. Ashe passive slows for 20-30% (30% at level 16) for 2 seconds. If she crits, it doubles to 40-60% for 2 seconds. Aphelios, because he doesnt have to level anything, just starts with 30% slow for 3.5 seconds, which decays to 10% after 0.7 seconds. So for 0.7 seconds he has a stronger slow than ashe, and then the slow lasts almost twice as long. Not only that, but Aphelios can combo that with his Q to turn that into a 1 second root. With his ult, he can apply multiple marks that he can turn into roots, making him a god tier engage ADC.
Flamethrower turns Aphelios into a fantastic waveclear and AoE ADC. His attacks basically get turned into ranged tiamat strikes. Crit strikes make said tiamat strike bigger and stronger. Jynx has to pay mana for her extended range aoe waveclear, sivir spends all of her mana early clearing waves, and then has to scale to be a threat as an aoe teamfight ADC. Aphelios, when he has flamethrower, just is. And when he flamethrower ults, he becomes an enormous threat to a teamfight happening in a small corridor.
The last gun, and no one understands this one, is chakram. It makes it so that Aphelios cannot auto until his chakram returns to him (cool concept) so he can became an insane DPS champ if he stands very close to his target. From the standard concept of an ADC, this is insanely cool and different, as usually ADCs want to be at max range from their target. But then their is also that Aphelios can stack chakrams, again, in a way that no one can really understand. And when he has a bunch of chakrams, he just flat out becomes one of the best DPS champs in the game, to match the likes of Kog, Twitch, and mini-gun jynx. You can have 300 armor and 3500+ health, all of a sudden Aphelios with 10+ chakrams kills you from 200 range in less than a second. 2 seconds prior he is dealing minimal damage with his sniper, but then he can just enter god mode with a new weapon. it also gives him heimer turrets which can do some other really weird shit.
In conclusion, Aphelios is a top tier example of garbage design, as he dabbles in every niche, yet when his is in that niche he is the best at said niche. The saying is "jack of all trades, master of none, but better than a master of one" Which means that they have tools for everything, but lose in certain circumstances. However they are more adaptable than the "master of one" Aphelios doesnt follow this mantra though, he is jack of all trades, master of all trades, and better than the master of one