The true reason that CertainlyT champions are unfun to play against.
People always say they are "overloaded", but that isn't exactly it, there are and have been plenty of arguably overloaded champions over the years not designed by him that don't get nearly as much hate. I think one word perfectly describes a strength almost all of his champions have:
Pressure
Every last one of his champions are designed in a way that they constantly demand your focus, particularly in lane. They are perpetually ready to tear you a massive new hole the moment you let your guard down for even a moment. They are designed to not give their opponent any breathing room.
Imagine shooting a 3 pointer alone on an empty basketball court. Now imagine that you have to do it in front of a crowd of thousands, you haven't eaten anything or slept all day, and there are armed soldiers on standby ready to slaughter a bunch of orphans holding puppies if you miss the shot. The latter is what it's like to play against a Zoe, pre-rework Darius, Yasuo, or new Graves.
High pressure champions are inherently stressful to play against, but CertainlyT champions essentially take it to a negative extreme for a lot of players.