I think this varies on a player to player basis.
Like, ADC mains play ADC because its what they are good at. Picking up something like Riven would probably be an awful experience for them since they don't have a full grasp on the mechanics, timing, and judgement of what to fight, when and how to fight, where to position etc etc.
From my perspective as a Top laner and Jungler, I view ADC as the hardest role in the game to play, but I doubt ADC mains see it that way.
My issue when I attempt to play ADC is, I don't know how to position to be effective, or when the hell I can even go in without dying.
And I don't know when im supposed to be farming or grouping. I also limit myself to very mechanically simplistic ADC's to minimize my mechanical problems at the role. Things like, MF and Twitch.
Been playing since season1, routinely land in Plat1-3 every season.
The only role I have never mained is ADC.
The only times I did play ADC, and did well, was when ADC junglers were viable.
Kog'maw Jungle I was strong with.
MF Jungle I was strong with.
Ezreal Jungle I was strong with.
Twitch Jungle I was strong with.
But its because I have a very solid understanding of how to maximize my gameplay from the jungle role.