Dusknoir has good stats and a good movepool, why is it tierless?
I asked on Smogon why Dusknoir is in the tier ZU (aka Untiered) on both the PU and ZU thread. I started with ZU. A user named Ho3nConfirm3d said:
Dusknoir did not reach the minimum usage in the past month to stay in PU, so it dropped to ZU. Specifically to the metagame, you'll find that ZU–and by extension PU–has better options in bulky Ghost-types like Cofagrigus, Jellicent, Trevenant, and more. Dusknoir simply has too much competition and not enough unique traits nor valuable stats to make it stick out. If you want to ask specifically why it's not used in PU, feel free to check out their forum :)
Source
From there I asked why Dusknoir wasn't used in PU. A user named Specs said:
It isn't PU by usage because it is outclassed as a ghost breaker and a ghost tank. Silvally-Ghost outclasses it as a ghost breaker, and the niche offensive Gourgeists. And defensively Palossand, Sableye, and Jellicent outclass it.
Basically, Dusknoir is Untiered because it is outclassed by other ghost type Pokemon both offensively and defensively. Hope this helps.
Outclassed by Eviolite Dusclops. That's why.