I think personally the biggest problem with Kaisa is that she has too few weakness for her too many advantages.
She can do %missing hp damage with her passive, like Jhin (and vayne(or does she just do a flat %of true damage?)) but unlike jhin she can proc her passive rather quickly due to allied CC and her attackspeed being able to be increased, whereas jhin has to fire 3 shots at a unchangeable fire rate until he gets his 4th. That with the reload means he only can do %missing hp damage every 4-5 seconds, whereas kaisa can do the same every 2 seconds.
She is also mostly immobile minus her e, like vayne, but unlike vayne can go invis once its evolved (which isnt tough) and doesnt require her ult to be active. Yes, she cant fire during this time, but her attack speed is then buffed after to compensate for the lack of damage, allowing her to further proc her passive even more.
For her to have a short range, she has a lot of ways to get close to someone, something that wouldnt be a problem if she also didnt have her e.
She has everything an adc needs to work very well. Her weakness is only “dont get close” which isnt always an option. Whereas adcs, like jihn,cait, kog and mf are rather immobile (lately jhin has been a speedy boi tho, so maybe less him) have a lot of damage but cant get out of situations as easily, someone like kaisa also has comperable (or better) damage without the lack of mobility. Then you have some adcs like vayne or jinx who has rather rough early games but scale well, and while kaisa has a medicire early game, she scaled just as well without the troubles vayne/jinx have.
But what do i know. Im only silver afterall :(