To be fair there's a lot of niche mons who can put some work in the tier, but for most of them it's because, as they are not common, people are less prepared to face them. An example to answer your question of niche Pokemon working well when their niche is fulfilled is all the Trick Room mons (Alolan Marowak, Cresselia, Porygon2...) even though Trick Room is not the strong playstyle.
Now, if I must cite a few niche Pokemon I found pretty good while testing, I would say that Toxtricity can be a niche pick with a Choice Specs set, as it's a pretty underated wallbreaker, able to threaten Landorus-T. Suicune is also pretty niche, but I think that it's an underated sweeper with its Sub/Protect/CM set. Last one I would cite is Aegislash, which can run a lot of different sets, going from defensive with Sub/Toxic to Specs or Swords Dance. In particular, I really like the defensive utility it can bring, being a pseudo switch-in to thinks like Tapu Lele. (this is only based on my own experience though)
Wobbuffet is completely useless in OU because Shadow Tag is banned, meaning you can only use it with Telepathy and against a good player, they will not fall for Counter or Mirror Coat and can just endlessly switch between their Pokemon and heal back their whole team while Wobbuffet seats there doing nothing.