Can you give examples of text that Word adds (or refuses to add).

Unfortunately, Word works very well combining documents that are formatted using well-thought-out styles and without direct formatting. It does poorly when one or more of the documents was written by someone who was using Word as a substitute for a typewriter.

Basic concepts of Microsoft Word: An introduction
Understanding Styles in Microsoft Word
Why use Microsoft Word’s built-in heading styles?

Unfortunately, few people are trained on how to use Word intelligently. Instead they are trained to get a document out that looks like they want, but may be a nightmare to edit.