Dusknoir is really underrated. Just because you use it doesn't mean you're a noob. Dusknoir may be outclassed as a wall in OU and may suck in RU, but when you use it as a SubPuncher, it suddenly becomes very good. I posted a team
that got 1935 ACRE on Pokemon Showdown with 19-9-1. Substitute/Focus Punch/Shadow Sneak/Will-o-Wisp is the way to go with Dusk.
I'm sorry, but no, Dusknoir still sucks horribly in OU. By the way you put it, it looks like the only reason you had any real success with it was merely the fact that people played badly against it, which proves nothing. Dusknoir has an incredible number of problems. First of all, it doesn't actually do anything except get a little surprise damage every once in a while. It's hard to say that it actually spinblocks when it often loses to the two best spinners in the tier. The only way that Dusknoir can avoid the 2HKO from offensive Starmie after SR is if runs Surf > Hydro Pump, has no Life Orb, has Natural Cure > Analytic, and is not in Rain. Otherwise, Starmie just 2HKOs (or OHKOs) Dusknoir on the switch-in. More defensive Rain teams often run SubToxic Tentacruel to beat Jellicent, and it can create Subs that survive Focus Punch and always break your Subs with a Rain-boosted Scald.
Dusknoir also doesn't actually threaten much when it comes in. Depending on the damage roll, Jolly Breloom only needs 4 or 5 Bullet Seed hits to KO Dusknoir. Celebi does pretty much whatever it wants since Shadow Sneak doesn't get even close to a 2HKO. Scarf Terrakion will usually 2HKO after SR with Stone Edge, and Band is even worse. Most Rain-boosted attacks coming off offensive Pokemon are going to be doing tons of damage, if not KOing altogether. Dragons just eat Dusknoir alive. That's just a few; Dusknoir does not succeed as a bulky offensive pivot at all.
Dusknoir also has problems with its stats themselves. For one thing, it's pretty weak. Base 100 Atk stat isn't that bad, but it's relying on the unreliable Focus Punch for its power since Shadow Sneak is just pitiful. This makes it very easy for many Pokemon to set up all over it and beat it, even if you can successfully force a switch and get a Substitute up. As mentioned before, SubToxic Tentacruel just sets up Subs all over it with ease. Poison Heal Gliscor laughs at everything it does. Gyarados with Sub or Taunt just sets up all over Dusknoir as well. BU Toxicroak plays mind games with you like you wouldn't believe. That, or it can just set up Subs until Focus Punch fails to break one (1/8 chance) and get up a Bulk Up, after which its Subs can't be one-shoted and it sets up all over you. Substitute Landorus-T plays with Dusknoir like Gyarados does, although it's admittedly rare. Shadow Sneak maxes out at 43.4% against SubCM Latias, so she just sets up on it as well unless you can predict around Sub with WoW perfectly. CM Reuniclus manhandles it because, again, Shadow Sneak is so weak. Venusaur can set up on it and heal back a lot of the damage that it does with Giga Drain. Any Volcarona with Roost sets up on it, and other sets without Roost can win and sweep depending on the circumstances.
Not only does it not do much damage, but it's not as bulky as its high defensive stats would lead you to believe due to its low base HP stat. It's barely bulkier than a Dragonite with equal investment (not counting Multiscale, of course), and it's less bulky than several common Pokemon such as Heatran, Jirachi, Celebi, Garchomp, and Conkeldurr (again, with equal investment) on one or both sides of the offensive spectrum. It also only has one common immunity and only one common resistance (if you even want to count Bug, since most of the time that's just U-turn, which is going to end up hurting Dusknoir in the end anyway). It also lacks reliable recovery to actually take advantage of what bulk it does have. All of this makes it very hard for Dusknoir to actually switch in and pose a threat unless the opponent is trapped into a Choice-locked Fighting move or something.
Honestly, there's not much that Dusknoir actually does that isn't done far better by something else. Its roles as a spin blocker and a bulky offensive pivot can be better performed by Pokemon such as Jellicent and Landorus-T, respectively. If you want a Substitute-based Ghost set that messes with its common counters and checks, just use SubWisp Gengar (which is a really cool set right now). Dusknoir just doesn't offer much of anything that I can't get from something else besides surprise value, which is never a good argument for viability. I'm glad you had success with it, since using a gimmick to success in OU is always fun, but all that means is that you outplayed bad opponents and/or got lucky. I've used plenty of gimmicks in various tiers myself (I once made the top 100 in OU on PO way back when with a team based around a QDPass Venomoth + SF Nidoking core), but just because you have success with a gimmick does not mean that it is actually good.