We have seen cases were excel files and word files don't read correctly. We have seen where an excel file is not properly read and large portions of the file are unavailable in the diff viewer. We ...
On the Excel item, all formatting is removed when viewing in the diff viewer, so I'm wondering if that's what you're referring to, or if that's possibly a symptom. If so, there is a separate request to preserve Excel formatting in the Collaborator diff viewer which can be found here.