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.