Most of the things you'd try to burn with Wisp, Golurk can already deal with just by punching their faces in. Iron Fist-boosted Ice Punch or Thunderpunch suffice in most situations where Wisp would save you from a Fighting-resistant poke (not that you should be SubPunching with Golurk since it has better things to do). If they dodge the Will-o-Wisp and set up Sub, there is little to nothing Dusknoir can do about it.
They also must resist Focus Punch and have some decent bulk (tentacruel).
1: Its neutrailty to Water does little for it considering most Water attacks it encounters are too strong for it to face since they are boosted by Rain, most of the Water types don't even care about Dusknoir anyway. If there is no Rain there is most likely Sand, which Golurk does not care about while Dusknoir cannot gain HP made from Subs. The lack of Grass weakness is a bit more relevant since Bullet Seed Breloom and Ferrothorn do exist, but isn't a outright resist much better? I'd rather try Breloom or Toxicroak, both of which even have 12% per turn recovery. 2: Its worse bulk is compensated by additional resistances (where the bulk would matter more) and greater offensive power. After all, isn't the point of a SubPuncher is to punish foes with "free" damage?
You're acting like each and every single team uses weather. Only 45% or something of all teams used weather in july. And Breloom and Toxicroak don't have access to will-o-wisp. Starmie is 2hkoed by shadow sneak. The only water-types that really don't care about Dusknoir are Jellicent and Tentacruel.
Who cares about typing, what are you even trying to accomplish with Dusknoir? Kill Fighting-weak pokes? Then the Fighters are obviously better due to STAB, not to mention they aren't stuck with a piss weak 40 base power STAB to back up Focus Punch. Landorus-T can overcome Will-o-Wisp with Swords Dance while you do nothing in return; if Gyara isn't hit on the switch (which may not be easily telegraphed), Dusknoir has to get out of there and lose momentum for you.
Many Fighting-weak Pokemon have no means of doing anything to Dusknoir (like ferrothorn, who's burned by will-o-wisp and can't do CRAP to Dusk if it has Gyro Ball). Swords Dance Lando-T isn't that common anyway, and Gyara's Sub is broken by Focus Punch unless it has intimidate.
Wait, now you claim Focus Punch is your coverage move?? So your main method to fight back is Shadow Sneak? By the looks of it, Dusknoir is almost completely dependant on unSTABed Focus Punch (lol) to do competent damage against neutral foes, while other SubPunchers can resort to their possible alternate STAB. We're not understimating Dusknoir's strength: 100 base Attack using an unSTABed 150 base power move is really unimpressive when you compare it to walls like Ferrothorn's 94 base Attack using 120 base power STAB Power Whip, or Hippowdon's 112 base Attack using 100 base power STAB Earthquake. If you can't hit harder than that with your main offensive move, you know your power is really subpar for an offensive poke.
Hippowdon is heavily crippled by Will-o-Wisp. And yes, Dusknoir can't really do much to things that do resist Focus Punch, which is why I nominated it for D rank, not S or A+.
You need to setup Sub on Ferro, which will easily break it with Power Whip, while going for Wisp means risking either Thunder Wave and/or Leech Seed. And another thing, Dusclops isn't really meant for taking on special attackers, but it can still attempt to outstall Latios with Pain Split after Draco Meteor. Dusknoir on the other hand, takes 89.79 - 106.12% (37.5% chance to OHKO, guaranteed after SR) from Specs Draco Metoer, so....have fun.
Not if I simply use Substitute as your ferrothorn uses Leech Seed, and then burn your Ferrothorn. Also, not very many things can take a Choice Specs Draco Meteor even without Stealth Rock, and Dusknoir's just going to switch in after it has already used Draco Meteor OR if it decides to use something like hidden power fire.
Dusclops can use Curse (which at least combos well with Pain Split) to deter setup, and the Volcarona example is poor considering you could get burned by Flame Body, can't Focus Punch again after it breaks your Sub, and loses to Roost variants anyway. The Magic Bouncers and Azelf are rare in OU, better examples would be Taunt Terrakion and Gyarados, the former of which you actually lose to if you fail to set up a Substitute.
Taunt Terrakion is very rare, and with proper prediction it loses to Dusk. I do agree that Taunt Gyara beats Dusknoir, however.
Also, you know that the definition of D rank says: "Pokemon who have a small niche in OU, but have very noticable flaws that make them more trouble then their worth the majority of the time." This is the perfect place for Dusknoir. It is outclassed as a defensive spinblocker, but not as a subpuncher.
To the post above, I agree with raikou for C rank. It has a lot of perks over Jolteon. For example, there's aura sphere to get past tyranitar but that conflicts with the fact that you have to run a rash nature. Raikou also has calm mind to attempt a late-game sweep after scarf landorus-t and hippowdon have been taken care of.