In Riot's own words, what they strive for is minimisation of burden of knowledge [as in, players should understand what an ability does after the enemy uses it without looking up the wiki.] CertainlyT is terrible at that. Armor penetration hidden in Yasuo's ult, CDR in Zyra's seeds, bonus infinitely scaling autoattack damage in Thresh's Flay passive, and so on.
Thresh and Kalista were both such big designs because they do just about everything other champs in their roles do. At one point in her design, Kal could globally teleport to her Sentinels if they were triggered, until someone rightfully told CertainlyT it was stupid.
I didn't blame him for Zed. But remember, Zed's been frequently picked/banned since his release, until the 1 second nerf to his ult "Get out of jail free" escape came along. CertainlyT was responsible for the addition of that escape.
Zyra, on her release, had a 65% winrate with people who had even had 5 games or less.
League of Legends has different champions with different strengths, weaknesses, and abilities because that's what makes the game interesting. But CertainlyT's kits seem to ignore that because he just packs them full of extra goodies which ultimately eliminate most of their weaknesses. It's like he looks at a champion kit he's making and then gets carried away, like they were letting a 12 year old design a champ. "Okay, I'll make a champion which can do this, this, this, this and this. Oh, but she should also have this! And this! And this thing!"
CertainlyT is the fan fiction writer of League champion design. His champions are Mary Sues, good at lots of things but with almost no weaknesses. Which is why they're popular. Why play a champ who actually has weaknesses?
Yes, other champion designers have made mistakes in the past. But every single one of CertainlyT's designs has been pick/ban some time after release. That's a very bad track record for a designer.