I need someone to explain me few things

Why is Kalista so hated? I'm trying to use her more often right now, because I can't play her as good as I'd like to, so I'm learning. The easiest ADCs for me are Varus and Draven, I can play really well with them, although I don't play ranked games this season (so I'm a normal game guy currently). So here are few things I've seen people complaining about: high dmg, mobility and control on her ult. The thing is, I find her dmg to be one of the lowest. What exactly am I doing wrong and how to play her more correctly? Is there some kind of a guide? Her "E" seems to be a good leg-up due to her low dmg (at least when I play her), but still, I have to survive enemy's Leona and Vayne somehow if I want to use it properly. I can agree about her mobility, yet characters like Vayne with their R/Q, Corki with his flying and other ADCs with dashes still leave me no choice, especially if they have Leona or Blitz who's "E" animation works even if you're 100 meters away from him. Her ultimate seems to be rather bad than good for me, because I can't always rely on throwing a support on them. There are ultimates like Ezreal's, Jinx's and others, which seem to be much more useful. Her passive could be really good, if my support would actually hit enemies in the face (what usually doesn't happen). So tell me, how do I, a pretty low skilled guy, should play Kalista if she gets even more nerfed? I need a guide or something, because I feel like characters such as Vayne or Leona will always be too strong against me.