As fiddlesticks, He is a mage, so any form of silence or channel interruption spell is his weakness. This is especially true if someone manages to interrupt his Ultimate R, as that is the ability which can single-handedly turn the tide of a team fight.

Grievous Wounds are quite effective against him. He is quite squishy as well so he won’t be able to do much if the enemy team gets healing reduction during the early game.

His biggest weakness is an enemy team that can ward well. This will legitimately counter Fiddlesticks entirely and will prevent him from all-in’ing the enemy team consistently. The vision will allow the enemy to anticipate his move and react accordingly. As fiddlesticks, It's especially true if someone manages to interrupt their Ultimate R, because it's the ability that can turn the tide of a team fight alone. The seriously painful injuries are quite effective against him. He is just as squishy as well as he was not able to do much if the enemy team gets a reduction of healing at the beginning of the game. His greatest weakness is an enemy team that can keep well. This is legitimately going against the Fiddles entirely and preventing him from doing everything the enemy team in a coherent way. The vision will allow the enemy to anticipate his movement and react accordingly.