I like Wukong and find playing him in casual play to be both fun and effective but I can see where he may not translate well to higher level play.
His strengths include burst damage and a strong team ult. The knockup and damage are both significant.
Wukong is lacking in the laning phase against most currently popular top lane champions. He does not have any form of crowd control until he gets his ult, he doesn't really have any range and as a result of these two things, he can get zoned. He also can't really harass very well against opponents that understand how Wukong works.
The standard Wukong combo is to Nimbus Strike in, Crushing Blow and use Decoy to prevent retaliation. That puts all three of his abilities on cooldown and him mostly within range of the enemy at the end of the Decoy invisibility. If the enemy recognizes this and proceed to walk toward where you will come out of Decoy, your invisibility will end and they will have just begun their combo which will likely include a slow/stun/root, ignite and whatever else they want to throw at you.
Now there is some trickery to do with Decoy like using the stop command to make it look like you used decoy. This tactic becomes less effective as the opponent's skill level increases.
I don't think that Wukong is weak but he just doesn't quite fit in the current meta or there are just stronger options since he doens't really hard-counter any champions.