(Unpopular Opinion Trigger Warning)
But, now that I've had enough exposure to the game, and looking at it from a fresh perspective, I've come to the realization that most of his designs are pretty smart. Say what you will about Darius, Yasuo, Akali, Zed... The man knows how to fit individual displays of skill into this team-based game.
The one thing in common with most of CertainlyT's designs is that they are straight-up ruthless toward their opponents, but also towards the person playing them. For example, I can't play Darius worth a crap right now. Now, most people have been on the receiving end of one of Darius's seemingly unstoppable rampages before, but have you ever watched a bad Darius get farmed over and over again, by something like a Nasus or the current iteration of Kayle? It's gruesome as hell, but you have to know that they (or I, in my case) deserve it. CertainlyT understands that big mistakes in lane like Flashing into a full-health Darius for a kill, without good reason, while you are already low shouldn't be met with a slap on the wrist, maybe a stray autoattack from your opponent. No, they should be punished the same way they would be punished in teamfights: with the offender's death, and an increased possibility that further mistakes in the future will lead to more deaths. Furthermore, if your entire team gets caught out by an Apprehend and then dies in a Noxian Guillotine slaughter... Well, you guys kind of had it coming. If Darius plays badly, however, he will be on the wrong end of the axe in seconds, so to speak.
Zed's design might be a bit harder to defend, especially toward bot lane mains. But know that as much as you want to carry and 1v5 the entire enemy team, he wants to as well, and if he's playing better than you on a champion with a higher skill floor, chances are he deserves it more.
For the most part, the champions that he had a hand in are not massively overpowered or anything like that, and most of them aren't pick/ban in competitive play these days (Akali was overpowered for awhile, but you can't pin all the blame on him for that one) But if you make obvious mistakes against a good Darius, Yasuo, or Akali, expect to be punished, and punished hard. However, know that they are in the same boat. If they screw up, and you punish them the way that they would punish you, they cannot expect mercy. All of these champions have 1v5 potential, but only if they punish mistakes you make without making any punishable mistakes themselves. Yeah, I get frustrated when I have to go up against a good one of these, especially if they are fed, but deep down I know that if one of these champions 1v5's your entire team, the side that played better won. CertainlyT, as much as I hate getting beaten down by one of your designs at times, I must salute you for your work. I hope I get to know their intricacies better in the future as I climb the ladder, and I look forward to what you come up with next.
EDIT: It turns out that my information on Zed was wrong. CertainlyT had a hand in it, but he was not the lead designer. Samizul was the lead, although as the source I looked up states, a lot of people had a hand in it. That being said, my point still stands that his work, specifically on champions he led design on, deserves some degree of respect. Even if you disagree with his design philosophy, CertainlyT puts thought into his work, that much is clear to me.