The quality of the message and it's impact are definitely influenced by the type medium but it does not follow that this makes the medium not neutral. Let's take an extreme case - a medium that when used will harm the receiver of the message. You might say that this medium is not neutral but the fact is that the medium is always neutral. What isn't neutral is the designer of that medium or the one who uses it. Even in the very unlikely case that both are ignorant of the harmful effects, the medium still remains neutral, much like a destructive force of nature is neutral.