Karma, Gragas, Riven
Karma - Subpar support with a decent shield but bad heal and extremely weak CC. As an AP carry, she falls off later into the game due to low range.
Gragas - Nerfed, high mana costs, high cooldowns, easily dodgeable. In team fights, he's just a "throw out your ultimate and try not to die" champion because of cooldowns. During laning, his mana costs are so high that he uses two Q's and is out of mana. He's forced to start with a Crystalline flask or mana potions because of this. His passive also only heals 2% for 1 skill. When you're Gragas midgame, you exchange 5 skills for 10% healing and then get oom.
Riven - Used to be quite strong and had great snowballing potential. Now, the League of Warmog's has begun. She used to be able to start with a BT, but now even with that, she can't kill a champion that's just gotten their Giant's Belt. This health-stacking meta has really affected Riven. Even if she responds by building health, she falls back a bit as punishment for not building some AD.