Illaoi needs lockdown to work in a teamfight, and hard disengage on the enemy team ruins her teamfighting
Zac is high sustain through offensive actions, as well as ramping damage threat. He lacks instant damage though, and if youre unable to get to your blobs youll likely lose trades.
Trundle is a great duelist, however if the enemy gets out of pillar and W field he has a hard time actually killing someone.
Ekko is reliant on outplaying the enemy; missing your stun/shield or being unable to assassinate someone can make you fall off in the fight.
Veigar is short range, and most of his damage is effectively single target. His only CC can also be avoided entirely. He relies heavily on his team to back him up after he uses his ult, and often if his ult doesnt kill someone he wont be supplying much.
Sivir is short ranged and objective based. Shes good at helping shift the momentum of a fight with her ult, allowing her team to out position or disengage or chase the enemy team, push lane or take objectives. Her overall damage in a teamfight however is relatively low compared to others, thus she generally doesnt want to teamfight unless the enemy mispositions.
Bard relies heavily on his Q and ult to land on something to make plays. Really his biggest thing is practice him. That being said, tahm does well against him.
Braum is reliant on being able to block one ability for his team, and being able to protect his carry. Someone like blitzcrank will likely do well against braum if he is unable to bodyblock, but AoE teamcomps likely hinder him. I dont see braum enough to really know honestly.