As an assassin, like Yi, you should be able to 1v5 and quite well at that but Nocturne just doesn't have that AOE damage like Yi's Q. That's why the extra health is almost necessary and even then you can get easily CC'd because of the slow movement so adding to the build having
could be great. He's just such a diverse champ and no one plays him like I would want to see. Almost every top tier champ is an ADC (Graves, Kog'maw) and not ones like Nocturne that have this. He can go tank and deal damage from the high scaling or go full AD and just not care. I bet AP could work as well since his E is 100% AP scaling and just have the team go in when he does.
Full AD Build
or

BotRK for Attack Speed and Lifesteal, DD for Damage and Lifesteal, Duskblade for the Kill Securing, Rageblade for that extra attack speed and AP, Either of the The Last Whisper items for the Armor Pen. and Phantom Dancer for chasing and Crits
Full Tank Build

Deadman's for the extra speed, health, and armor. Spirit Visage increases your passive, BT for that great Lifesteal and damage as Nocturne, BlackCleaver for the damage and health, same with the hydra but with the on-hit passive, and Sterak's Gage for health, increased base damage, and the passive.
Whatever you want to build, it works. But people don't want to play him. He is one of, if not the only, champion that I can call my favorite.