Without looking into the details I would guess that MDS does bump into a member of Concept (namely ConceptAttribute) that it doesn’t know yet how to handle, and this fails the process altogether. Which makes sense since concept attributes and concept attribute types are fairly recent in OpenMRS.
@raff to confirm.
If I’m right, then someone would have to go and expand on MDS for it to know how to handle this new part of the Concept data model.
As I happen to know your need and use case I would suggest that you wait for this to be ready enough to be tested and used, rather than investing time and resource in expanding on MDS (even though that would be nice too!)