CC-BY-ND is, indeed, not an open license, and that’s the point. It’s one of the two most restrictive options listed in the linked issue, the other being “a custom license, granting rights to copy and modify for the sole purpose of contributing to our projects”.
Under discussion here is a content license for project assets (graphic design/artwork, use cases), not for software - as that’s already covered in another thread: What software license should we use?.
With that out of the way, my recommendation would be CC-BY-SA for functional design documents and specs, and CC-BY-ND for graphic design/artwork. The rationale being, we can expect derivatives for specs documents (text, diagrams) but not for actual creative “brand assets” work - I suppose.