He's very awkward. If you can go mid and get a Dagon fast, and if there are no BKBs so that you can actually use the Vendetta hit (rather than leading with the stun), then he can snowball hard and be an absolute terror to supports. Vendetta, stun, Dagon, dead.
But if those early kills end up limping away on 50 hp? He's... a stun. Niche utility against Medusa and OD (and perhaps Silencer?) because of the mana burn. And invis is always useful for scouting. But he's not Nyx Scout. He's Nyx Assassin, and that means he needs to be able to assassinate people.
I wonder if he'd be better as an int hero. He naturally wants int items (his spells have a pretty high mana cost, and you want to be able to use stuns and burns on cooldown in teamfights), so I wonder if it'd help give him the extra damage he really needs.