I find him hard to carry a bad team with, but he is certainly very strong.
play him more, get even better with him. He can carry a game with low elo scrubs at diamond.
He really, really can't if the other team builds tanky. I've experienced this so much. You can have any full build you want, with Deathcap, Void Staff, Zhonya's, any amount of flat pen or tankiness or spell vamp... it won't matter if the other team just builds tanky and CCs you to keep you away from the AD carry. Especially one that has an escape to get out of your ult. Now, if your team is slightly worse but you're still able to actually get them together for a teamfight, you can carry, sure. But if this is the kind of team where everyone's constantly getting caught as soon as laning ends, you're screwed.
There is absolutely nothing you can do as a short-range character with only a speed boost to get past a Xin Zhao with a frozen mallet to reach the Ezreal with Iceborn Gauntlet or the Corki in the back. If that carry is smart enough to stay out of harpoon range before the fight starts, you just can't get to him without a decent team backing you up. --- "That is why war is so tragic. To win means to make victims of your opponents and give birth to hatred." - Kratos Aurion, Tales of Symphonia