Olaf can delete entire teams by himself...if he gets super lucky Glacial procs, and the correct items.
Poison can also keep him from ulting for a long time, Predator kills him before he can ult, especially Poison/Predator (a large part of why Poison/Pred is meta right now) Mages can essentially just straight up delete him unless someone gives him dragon claw, and unless he has both Rageblade AND BT, a good Blademaster or Light comp can outduel him ez pz. And of course, even with Rageblade/BT/DClaw, a good assassin team at that point is still gonna crit him twice and end him.
The thing that needs the biggest nerf in all of TFT right now is Veigar - tier 3 Veigar deletes entire teams on his own, and doesn't really even require good items. Just 6 mages. Which you're gonna have anyway if you're running him, and you'll probably also have at least 2 shadow bonus so even on other Tier 3 units he'll hit like a train.