The server-to-server specification was only formally released on February 5 of this year. Before that, it was certainly necessary to do reverse-engineering of synapse in order to build a HS, but it no longer is. The lack of server implementations just over a month after the release of the spec isn't good evidence of a bad spec. I strongly disagree with Jason's (who has been openly hostile to most of the matrix community including me) characterization of the spec in its current form.

Disclosure: I like matrix and have hacked on a few matrix-related projects as a hobby activity (Jason would refer to this as buying into Matthew's (the CEO of New Vector, the company primarily driving matrix) personal cult). That's it.