This interests me also.

It's my impression, from both the license-list explanation
and the actual list, that SPDX casts a broader net than
either OSI or FSF. Substantial compliance is sufficient.

I also note:

The SPDX Legal Team endeavors to explain its reasoning,
analysis, and conclusions with respect to a candidate
license as a means of developing precedent.

Compare and contrast a very recent conversation on OSI's
license-review, starting here:

http://lists.opensource.org/pipermail/license-review_lists.opensource.org/2018-September/003535.html

--
Kyle Mitchell, attorney // Oakland // (510) 712 - 0933