well, i think the mechanical difficulty in league is very low and basically everyone with two functional hands can easily have faker-mechanics when looking at the speed of pressing buttons alone. in warcraft 3, many players had 200+ apm (actions per minute) including myself and in league its like, you got nothing to do really. in warcraft (or starcraft) apm spikes to 600-1000 per minute during combat, in league combat it barely spikes because well, you have like 3 buttons to press. every kid can do it.
what makes faker so unique is his gamesense and knowledge so he doesnt just press buttons but he actually uses his strengths at exactly the time and place he wants to use them, THAT is what makes him mechanically proficient, and not that he can swap zeds shadows around fast.
like, this "legendary faker zed" video where he completely obliterates someone in a zed mirror duel clearly shows that he actually doesnt react to the enemy movements but rather predicts them. he doesnt flash the shuriken, he flashes in the moment that his enemy would have to throw his shuriken in order to kill him so if he throws them, they get dodged and if he doesnt throw them, he still loses the duel. this is gamesense and this leads to good mechanics. its not that faker has some sort of super human reaction timing, even tho this surely helps too.
"mechanical difficulty" doesnt really exist in league. thats why basically all champions are equally hard to play, even tho there are some differences but they are not very grave. and sure, for some people, some champions might be harder than others simply by not being their style or type or preference. for me, rengar is super hard to play. i suck with him so bad it ashames me, but i wouldnt say rengar is an incredibly hard to play champion.
also, i agree on the point that champions who forgive mistakes more easily are generally easier to play by a bit. for me, those champions do ACTUALLY include nidalee, lee sin, zed, fizz and the likes. they are a BIT easier to play.