The Voynich Manuscript is a mysterious 15th century vellum book full of bizarre drawings, mostly of plants and women, and text written in an unknown language or code. Explanations include a coded book of health advice for wealthy women, a work of religious mania, fantasy or heresy, or simply a hoax. Even mundane proposals can get a rigorous mauling, such is the intensity of interest. Noticing similarities between it and the work of mentally ill artists (and echoing speculations from an earlier book by Gerry Kennedy and Rob Churchill) Carl Svensson steps into the fray: perhaps it was created by someone with schizophrenia.
The above examples – just two out of a plethora of similar art created by people suffering from schizophrenia – lead me to believe that the Voynich manuscript cannot be decoded because it contains nothing that's possible to decode. The text, like Charles Crumb's almost-cursive, follows some kind of pseudo rules a literate person might instinctively apply. This makes it look like text, feel like text and ostensibly carry information like text but is, in the end, a hopeless attempt at conveying something without the rules and rigor of actual language. The illustrations are, just like Hill's later drawings, repetitions of themes found in other books, filtered through the same graphomaniacal filter as the text, though emerging as something slightly more approachable.
The image up top compares a page of Voynich with a work by Carl Fredrik Hill, known to have lived with schizophrenia.
As a layman with familiarity with the subject matter, the stuff noting odd patterns of people/faces/animals is quite convincing. That said, Hill's painting is a swirl of impressions, an uncanny mess, whereas Voynich has a strong idea of the forms it represents and orders. (In this regard, Hill's painting reminds me of art generated by machine learning AIs, which have no apperception of reality)
And the Charles Crumb stuff about compulsively generating meaningless pseudo-text doesn't even get warm, for me. The consensus is that Voynich's text has features of natural language, including brevity, that would exclude that particular kind of disordered logorrhea.
This makes me wonder if the Voynich pictures and the Voynich text are by two different people: an outsider artist and an insider writer, if you like.