I wanted to chime in as another developer in the server area. I've recently been working on a service that sits atop of an existing homeserver (like Synapse) to handle outgoing federation in .NET rather than Python (which has proven to give quite an nice performance boost). The project implements it's own federation and signing code.
So far I've had to do almost no special casing around Synapse and followed the new S2S spec without issues, which has all been done in the open. The special casing I did have to do was brought before the team, judged a Synapse bug and will be fixed in an upcoming update.
I don't have any evidence to point to whether the spec can be used to write a complete homeserver implementation, but everything so far has been done painlessly following the docs online. I believe that I could, with enough time.
The project is developed by myself and another, and we've experienced no hostility from the team when asking questions about the protocol or the above bug. Quite the contrary, they were quick to help and the project has been promoted by matrix.org every week in the news blog posts.
On the project itself, we hope one day to branch out from working on services that sit on top, to being a standalone homeserver (which should progress naturally. I don't foresee any spec related problems with doing this from reading the spec and chatting to folks in the community.
So I felt it would be worth sharing my experiences as another server developer, albeit one that hasn't gotten as far in their journey yet.