I think his clearest weakness is his complete inability to deal with range at all.
He has one of the SLOWEST "movespeed" buffs in the game. Not only is it not instant for the speed, but it has a pitiful shield that breaks the speed buff.
Any ranged champion with cc will obliterate him before he can ever do anything. He can't even reach people he slows most of the time because of how little his W hastes him.
On top of that, there is absolutely no reason that skarner should not be able to auto attack the person he is ulting. The person is literally being held right in front of his claws. THIS IS STUPID. This makes him lose a lot of fights simply because his cc is tied to a self stun in a 1v1 situation.
No other fighter/assassin basically suppresses 75% of their kit to hit the enemy.
Especially since skarner is not cc immune during the ult. Think about that. Many will say "skarner being cc immune would be broken" but forget that a full range blitzcrank pull will cover more distance and cant be interrupted once it hits.
Skarner has 1.75 seconds to drag someone AFTER he gets into melee and can be stunned for the entire duration of his ult.
Something needs to be done so skarner isn't punishing skarner for making normal plays."haste yourself to gank, lose shield for the actual fight." "Ult the enemy carry, suppress yourself so you can't chain cc them with E or auto them." "Need to hit hard with Q, first hit soft as hell with Q then wait 4 seconds to do regular damage." "Need attack speed to be relevent? Enemy never fights in the jungle to make sure you never have said advantage."