Q:
Isn't our product too complex for an external author to understand?

A:
You may think that your staff know the product better than anyone else, in fact they probably do, but that doesn't mean that they are the best people to explain this to novice users.
  • Your own experts will assume knowledge that the user may not have

  • They will have a tendency to over explain the technical wizardry of the product because that what gets them up in the morning

  • They may use industry "jargon" without even realising it

In short they may talk about the leaves on the tree, rather than the forest.

This is not to say that you won't get a reasonable job done by your own staff, it is simply that the user might better understand the manual if it was written by someone without any preconceptions and with a novice's point of view.