Fair comparison
Universal Document vs email attachments
Email attachments move copies well but usually lose authoritative-state context. UD can carry explicit lineage and current-state relationships with the object.
Not a forced replacement
UD is designed to coexist with established formats. The practical question is which object should carry structure, state and lineage, and which formats should remain compatibility exports or required delivery formats.
Evaluate with one real workflow
Compare how each approach handles revision, finality, portability, machine readability, provenance and recipient friction.