[Pneumaticat]

At least from my perspective (I've never implemented the spec), I wouldn't call it just documentation [1]. Sure, there are likely some holes, but overall it seems like they've put a pretty good-faith effort into making it detailed enough for someone to implement.

I feel like the difficulty comes mainly from the breadth/size of the spec. Which is a major issue (the Matrix.org team's own alternative server, Dendrite, has been slow to progress as well), but ultimately I'm not sure what they can really do about that considering one of their key tenets is to contain all the functionality out-of-the-box, so to speak (no extensions like XMPP).

[1]: https://matrix.org/docs/spec/server_server/latest.html