Awesome 1v1 but as soon as teamfights begin his lack of mobility puts him at the back, and his lack of AP takes away all the damage from his poke. You don't even need CC, just one champ to take the stun, and the other to kill him. He can't kill faster than he is killed in 2v2, so he requires a super good peeler (like braum) but even braum has limits, in 5v5 it's too easy for one of the damage dealers, especially bruisers or tanks to get within range, his strength comes from that annoyingly powerful poke, and his Ulti hunt and yellow card combo on SOLO targets
Also, he can't chase as easily as many adc (with long range ad poke like Cait or ms steroid like Vayne) so in 1v1 you can just run away most of the time, unless he brush ambushes you or gets a good Ulti
Sometimes a strong champ is still weak, because the "situation" and "counter-play" options, you rarely get in your ideal "zone" where you can apply all that damage. I went AS/AP Ivern and solo'd baron at level 14, so strong, but that build against other champions is so bad due to his kit and how easy you can work around his "ideal damage situation"