I see no reason to ever play Nocturne when Vi and Rengar exist
And that's not a knock (or noc for the phreakish puns) on Rengar or Vi as champions. But the fact of the matter is, they do his job and do it significantly better than he does for a few reasons.
Let's start with Vi:
She has more durability More upfront damage and good sustained damage, not to mention utility in the form of armor shred Better ganking at basically all points of the game Better clear in the jungle Functions better as a bruiser/off-tank (keep in mind whenever nocturne has been played competitively, he's been built basically full tank + botrk) Arguably, she scales better than Nocturne
With Rengar, there's not a lot to say. Rengar is just a better version of nocturne when you build both to do damage, whereas Vi is better as a bruiser/off-tank and more reliable overall.
Nocturne needs changes and I'm not sure buffing numbers changes much, unless you just overtune the crap out of him, which is not the way to go. He needs his role to be differentiated somehow. Paranoia is his bread and butter as a champion and the spell has a lot of power in it, given that it denies vision ontop of instant dive. But the fact remains he can seldom use the power of his dive. Obviously his E has a delay on it due to the simple fact it would be kind of silly if he could ult from such a range and instantly stun somebody, but Vi does this from a pretty respectable range, too. Minus the semi global range obviously.
I'm not really sure what the answers are, but nocturne just kinda seems lackluster compared to those two. His spell shield is also a pretty weak defensive tool. And his base defensive stats are not good. And his passive is honestly kind of bleh.
I know sated devourer does help him some and is good on him but at it's core, it doesn't solve any of his problems. He just needs some power shifts I suspect, maybe a complete removal of his W and a new skill entirely or something. But either way, he definitely just feels pretty lackluster
is FOTM? Because he's held the highest play rate in the game for as long as I can remember. And yet, I would never call him FOTM. Why? Precisely because he's been extremely popular for ages. Same with Lee Sin and Riven. Vayne has held that 35% play rate for about 4-5 months now; it's clearly not just a fad that will go away on it's own. The only way Vayne's pick rate goes down is if Riot nerfs her. Hell, Tristana is a far better example of FOTM than Vayne is even if she's less popular; her pick rate went from 6% to 16% in the last 4 patches. Popular ≠ FOTM

and
: POWERSLAM!