They have very well defined documentation for all the stable parts of the API, their code is modern and quite readable. 
In addition to this Lennart is good at publishing lengthy explanatory blogposts aimed at system administrators. 
Finally they have done a truly admirable job building a community and explaining the why&#x27;s and how&#x27;s of their project. This is evident in the fact that there is now a systemd conference coming up.<p>If this fails the test, I suggest that it is a marvelous failure which every project should strive to emulate. 
To suggest that systemd is poorly documented indicates an insufficient exposure to the sad reality of most Open Source projects where &quot;our unmaintained, outdated, bug riddled code base is the documentation&quot; is the best case answer to the question of documentation.