annie is also bullshit. her point and click stun makes her extremely hard to balance.
complex kits and these mini games revolving around them give these champions room for error, which means they can be appropriately powerful when used correctly, and they can be outplayed at the same time.
abilities like annies ranged point and click stun on her q are too reliable to allow them to be too powerful. which means they have to be nerfed enough that it makes the champion feel like crap to play, because she constantly gets outplayed and outperformed by champions with more complex kits.
simple kits can be fun to play and they can be balanced. but annie is a bad example. we also already have a lot of champions with simple kits in this game. there is a limit to how much variety you can create with the same mechanics before the champions start to feel the same.
we had this problem in season 2, when they used to only make simple kits and release one champion every 2 weeks. they started to feel like copy pastas of each other.
new champion x has a static passive, 1 new ability that still somehow resembles something existing, and the rest taken from 3 other champions. or something incredibly generic like an empowered auto attack on his q with an on hit effect.
by making champion kits more complex, it makes it easier for riot to make the champions feel vastly different to play, and give each of them their own unique nieche. for example you wont find another champion who does the same things azir does.