Question about unicity distance

[Rafal]

So, you need about 140 characters of the ciphertext to identify the key for an English plaintext using this encoding scheme.

I'm not familiar with that measure but this link You are not allowed to view links. Register or Login to view.  says:

Note that just because you have 30 characters of ciphertext for a substitution cipher, this does not mean you will be able to break it. The Unicity distance is a theoretical minimum. In general you may need 4 times more characters (100 or more) to reliably break substitution ciphers. The same problem exists with the other ciphers.

I also have a feeling that this value depends on the type of the cipher used. As we don't know the type of cipher used in VM, then we don't know how to count it. We don't even know the language which is probably a problem as well.

But Voynich Manuscript is really long. Common sense tells us that with this amount of text it should be easily broken. Yet it isn't.