Otherside Picnic, MH
One thing I dig about Otherside Picnic is the way it handles the whole "unfathomable horrors causing you to go mad". Namely because seeing a big octopus isn't going to break my brain in any way, even if it might be scary.
However, the Otherside alters your cognition, it twists pattern recognition and other things in a way that feels consistent with actual altered states of mind due to psychadelics, etc. things might take on sinister meanings, for instance, because your brain is firing all its pattern recognition circuits on innocuous things.
Having said that, Sorawo (and Toriko, but the books are from Sorawo's perspective) actually DOES have some serious mental health issues, for instance, dissociative episodes due to past trauma.
Otherside Picnic, MH
In fact, she tends to dismiss her past trauma as a normal occurrence. She also has dependency issues (both of them, really) and her misanthropy borders on sociopathic sometimes... but that could also be her being an unreliable narrator. The point is that both of them are very fucked up and while their exploration of the Otherside brings them closer together, it's almost more of a coping mechanism than anything because, at least at this point in the novels, Sorawo hasn't really actually properly confronted her experiences.
Otherside Picnic, MH
In other words, ultimately it's about two queer women using altered states of mind to avoid dealing with their past trauma, so of course it's the sort of thing I think is compelling.