Anivia has a high leaning curve. Played in the right hands she will carry a game. She's the type of champion where you get better by knowing how to play the game more than knowing the champion. Being good on her wont mean squat if you don't know what to do after you kill the enemy. Or understand the enemy enough to not die. She has a few ways to handle mobility. But so many of them require perfection. Her wall's center has a mini knockback effect that can disrupt dashes. Her stun is near instant close range for other gap closers. She has some of the bet kiting in the game. Her damage is strong, only really held back by her mana costs.
Malz is very strong but niche. Mobility isn't his weakness. He actually has tools to counter it. His weakness is that if he doesn't win mid game will rarely land an ult for the rest of the game from enemy CC. The power he has comes with limitations. He doesn't get played by pro's because they understand those limitations. Even if he gets a favorable pick he will get camped all game by every lane. Bot will push and roam. So will top. While he has strong waveclear, he has to be ahead to use that. He has to be ahead to zone the enemy while clearing forever with nothing stopping him. Once he is zoned out it's hard for him to get back in.