I've posted this before on Nick's blog, but the diplomatic ciphers have HUGE character sets because they include both one-to-many, many-to-one, and many null characters (plus a glossary of characters for commonly used names like that of the pope or the current emperor).
A single cipher sometimes has more than 200 glyphs and each cipher is given a new set, resulting in thousands of different glyphs.
In contrast, Voynichese has a very limited glyph set, even compared to some natural-language alphabets.
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The diplomatic ciphers are extremely varied in where a glyph can occur in a word, partly due to the one-to-many/many-to-one structure and partly because it's a substitution cipher.
In contrast, Voynichese glyphs have very constrained positions within tokens.
These are two very significant differences.
A single cipher sometimes has more than 200 glyphs and each cipher is given a new set, resulting in thousands of different glyphs.
In contrast, Voynichese has a very limited glyph set, even compared to some natural-language alphabets.
.
The diplomatic ciphers are extremely varied in where a glyph can occur in a word, partly due to the one-to-many/many-to-one structure and partly because it's a substitution cipher.
In contrast, Voynichese glyphs have very constrained positions within tokens.
These are two very significant differences.