Interesting question. I’m kind of regretting not actually questing through most of Hordeside Draenor and doing most of my leveling through dungeons now, because I can’t say with certainty if any Forsaken are shown being affected by the Botani (or if there’s anything addressing that).
My immediate response would be ‘Yes, they are immune’, due to the fact that they have that necrotic energy running through them, which I think would exterminate any sort of life form that isn’t specifically acclimated to it (bloodworms, etc.). Unlike Forsaken, they aren’t just undead— they’re weapons meant to destroy life through frost and disease. A typical undead’s body might be a fine breeding ground for the infestation (nice and mulchy!), but something like a death knight or other plague-bearer might be too dangerous. If they’re not completely immune, they at least have a strong resistance due to their polarizing nature— life vs. death, and all that.
It also seems to have a gripping effect on the brain of what its controlling, but if something’s thinking power and consciousness is no longer located in the brain (like an undead creature*), would it have any control if it could infect them? Would it just infest the body but then not go anywhere because that body’s consciousness is not physical but rather spiritual/magical? Would it just be an inconvenient infestation of their body that doesn’t really hinder them (but probably isolates them so they don’t spread it)?
I’d like to know if there are any quests talking about the Botani and the Forsaken, because even though they’re a bit different from DKs, it might offer a bit of insight that I don’t have.
(*there are a lot of jokes about brain rot but honestly, if someone’s dead and resuscitated even after too many minutes passing they have irreparable brain damage. Since undead are clearly not unintelligible clumsy vegetables, I assume their consciousness/mind comes from their soul instead of their brain)