One of the few champions that was not very used in lines, so his strength against other champions melees combine him in a way to abuse those tankes or assassins that with certain skilles lower you. Thanks to its W being a skill too useful for all situations as a banshee allows you to always enter any tf.

Nocturne was one of the champions I use in season 3 with kha mid before the nerf and still remain very powerful almost againts any melee champion

Pros
-His passive is the most powerful in low lvl
-high attack speed
-good roaming potential
-can win almost every melee champions
-at lvl 3 can fight with a mage
-Good escape with the Q

Cons
-Range mage can easy trade with him
-you need to know when to escape because he have low hp
-in late is hard to kill the adc solo, just because the enemy team will not be alone
-carry is hard just because he fed its not easy to win solo (I was in that situation many times)

how to counteract
-if you face a mage just farm, if he/she do many damage use you passive with many minions. Also if you go foward he will think that you will attack him but nope.
-If you can't kill someone with the combo R-Q-E-Attack, just run because you will need more damage. if you use the R against any enemy be sure to have some way to escape.
-Try to give kills to the team if you are very fed that make the game more balance to can win in team