Kalista's passive is not op
I mean lets all calm down for a minute and think about it. Kalista plays in a very similar way to ashe. They both are excessively good at kiting but if you get to them they die. So if just being able to kite is a problem why is ashe not a problem? The answer is because the rest of the kit is quite different. In regards to her passive her base atk speed and atk speed/lvl is to high. It allows her to itemize damage and can potentially let her move around too much. or we could take issue with the really high 20% burst on her w that makes her a lane bully but because her e and q scale so well she has a good time in the late game. However complaining about the passive is kind of silly and won't really make her much better to play against.