Yi and Irellia are both strong late game champs, but they can also snowball into lategame, particularly Yi. If you're not confident that you and your team can either shut them down or not feed them at all, then letting them in the game can cost you the game,
Darius has always been a strong early game champion against melee, though I've noticed he's been played a lot in pro games lately, with a lot of people giving him Tons of Damage instead of the usual Black Clever. I want to call him a noob killer, but he has been pretty strong right now. I've seen even Faker playing him recently.
Yasuo and Zed are straight up noob killers. They do have a high skill ceiling and can be smart picks at all ranks of play, but generally in lower tier games and blind picks, they'll get banned because it's easier than actually learning how to fight them since they have good pokes and strong all-ins. I myself have long since learned how to shit all over Yasuo, but I will admit that I do still ban Zed sometimes just because he does hurt a lot of mid-lane mages anyway.