I don't get your point. Since IIRC Office 2007, docx is the default format which is designed for compatibility. IIRC it allows embedding of fonts and other non-text content, making compatibility concerns an edge case. If all parties use relatively recent version of MS Office with overlapping featureset, there should be no compatibility concerns, unless somewhere in the collaborative pipeline you are departing Office ecosystem altogether.

I don't think discussion around fault is meaningful in such scenario altogether.