I have a couple .docx documents to which I've added a few merge fields. When I MailMerge against the documents, the document layout completely changes, section headers get right shifted, underlining removed, form checkboxes vanish, etc.
I've hacked up the mail merge project from here and attached it to demonstrate what I mean (you'll need to remove the .png extension to revert it to a .zip). If you compare Doc1.docx and Doc2.docx to their mail merged versions, you'll see what I mean.
Is this something that's just unsupported or am I doing it wrong?