My adc mechanics are shit, but I'm switching to mid/adc from mid/supp
When I'm consistently 3-5 levels behind the top and mid laners, I simply can't make the kind of difference I need to in order to carry (and when I say carry, I generally mean "empower an ally so they can carry with ease").
At least as an adc I can bring wave control and objective focus.
Edit: For example, here's an (unranked) game I played adc. I (sivir) was always late to teamfights (most of my kills are from the very end) and just pushed side lanes and took towers, and we won the game with me having double Jhin's cs. Had nothing to do with my mechanical prowess (I was tired as hell and had no reaction times), I just made the right decisions.