Let me clarify my Janna play. I crush lane most of the time, because I know how to take advantage of her AA - W range and poke the enemy ADC and Support out of lane, thus creating big pressure. Maxing W works wonders. It might be easy to do, but you won't see other Jannas in Plat max W. They all max E and afk. Since I'm responsible for creating a lead bot (my ADC usually afk farms), we take first tower and rotate to top side and follow the basic winning strategy of this meta. And it works really well. Sometimes, in order to force my team to pressure something that is worth (like pushing a minion wave to secure an easy tower, when they won't), means that I will put myself in a dangerous position to apply said plan. Sometimes, it leads to my death, but the setup is good and my team still gets a big advantage out of it. If this ruins my KDA, but increases my team's advantage, then I will do it 100%. Also, there are games where we are losing hard, because some top laner goes 0/5 in 10 minutes. At that point, I know if I play my regular style, we will just lose, so I need to make a risky play to try and get an advantage. Sometimes, it won't work and I will die for it. I could honestly just stay under tower and deny my tower falling down, while I watch other lane towers fall to preserve my KDA. Since I value my win rate more, I'd rather take a risk. That's to clarify how I play Janna and why I don't have 2 death avg. I wish I could send some replays. Also, as a sidenote, I deny most Lee-Q-esque dashes with my tornado and me general timings on Q and R are good, so her only mechanical parts of her kit are applied in really good success rate.
Now, moving on, I do agree that ADC is the hardest role in the game, because you have so limited options as to what you can do with your kit, yet so much is expected out of you, therefore getting good at those mechanics is more than important. You can't be a good ADC for example, if you can't kite properly, but you can be a good mid laner, even if you can't aim your abilities 100% of the time.
Caitlyn is one of the hardest ADCs. Not as much mechanically, like Kalista or Draven or Ezreal, but because she has a very clear way to playing out her kit, that if you mess up and not gain the advantages you can get, you are screwed. If you don't get a lane lead and first tower as Cait, you either fucked up somewhere, or you just got hardcamped by the jungler. If you don't do that, then you are almost useless up until much later. The way that Cait gets this advantage is very simple, but also requires a lot of practice to pull off properly. Playing around your traps and your AA range sure seems simple, but it's not as simple. If you're unable to constantly abuse your extra AA range and zone people off with traps and chip down the towers, then you are doing it wrong.
Tristana is on the easier side. She has a good escape tool, which you should know when and where to use, which is the case with most champions with a dash. Her only real unique mechanic skill is being able to proc the bomb for a jump reset, which isn't really that hard.
Sivir is honestly one the hardest side of the ADCs, if you ask me. Lowest range + no repositioning spells means that you have to be very careful. Knowing when to use your spell shield + perfecting the W AA reset + being able to use the MS boosts to kite properly takes a lot of practice, and since she generally doesn't build a lot of attack speed, you need to orbwalk much better. Kiting as Sivir is harder since you are forced to do it more often (low AA range) as opposed to a Jinx who can just sit there and fire off autos. Sivir is much harder to maximize your DPS than most carries. The only other champions that are hard to maximize damage output are Ezreal and Draven , for different reasons, which is why you don't see good Dravens or Ezreals up until much higher elo.
My KDAs as Sivir might not be as good, because 1) I played some games with her to practice after a long time, so I didn't perform as well and 2) same with Janna, sometimes I must do the risky play, so I die more often. I could preserve my KDAs.
Also, generally, I find it harder to keep good KDAs on ADCs and supports. Seeing how you rely so much on your other ally laner, they can break your laning phase and make you look like a monkey. You can see how I had much better KDAs in last season for example with Maokai and the season before that with Viktor.