In medieval Europe some topics where just too risky for a person to write about (see Keagan Brewer's excellent overview on encrypted gynaecological or sexological texts in the time period the Voynich manuscript was written).

Yes, a lot of plants seem hard to identify, but so are other illustrations in other medieval manuscripts as well. And just think of a person copying an illustration from another source without any botanical background. Usually manuscripts were illuminated by specialists, but obviously including a third-party professional wouldn't have worked out if the author/copyist wanted to have it secret. So he or she went for the (less skilled) in-house option. The quality of the illustration is clearly way below the usual quality in herbaria of the time.

What also is often forgotten: Things vanish in hundreds of years. We just see a single manuscript here. No one knows if there used to be a couple of codices in this writing system and a small group of person was used to work with it.

https://ceur-ws.org/Vol-3313/paper16.pdf

Yes, some papers elaborate on this, but the most recent studies on this (like the ones presented in November on the International Voynich conference) make clear that it is not some man-made gibberish.

It also makes absolutely no sense to spend such amount of time (and parchment) for such a hoax in the early 15th century. Who would be the audience? An early modern hoax made by alchemists to swindle Rudolph? This sounds imaginable, but not for the Voynich Manuscript which is without doubt a product of the early 15th century.

Yes, there are still thousands of letters of physicians and scholars scattered in archives through Europe that have not been indexed. I believe that there might be a letter out there where Carl Widemann or someone else is mentioning the manuscript who certainly already looked interesting to a 16th century scholar. I spend almost two years and tracked down a lot of previous unknown letters but without any direct mention so far.

No, really a LOT of statistical analysis on the writing has been done in the last year which rule out that the text was just meaningless gibberish. It has properties of a language, but it could either be an an unknown language, an unknown short hand system, an unknown cipher or a mixture of these (the latter impossible to crack).

Analysis of the book binding, the illustrations and some color annotations in German narrow down the area to the wider Alpine region. Could be southern Germany, northern Italy, hard to say.

In my opinion the content is much less spectacular than most people think: A recent study shows that is probably dealing with "women medicine", i.e. fertility, abortion, sexuality etc. There a numerous examples that these topics have been censored or encrypted in manuscripts of the time.

I recently published the outcome of a comprehensive research in several archives in Europe and found what I've been looking for – a possible previous owner of the Voynich manuscript which may help to track down the provenance further.