Self-hosted CI on the other hand requires a lot of resources, especially when you're not only testing Linux or BSD, but proprietary OSes (OSX, Windows, although the latter at least has the Edge images that work everywhere).
Ultimately I feel like the use of CI in many open source projects isn't very high due to constant annoyances and difficulty debugging the CI environment.
My perspective may be driven by the communities I'm involved in, but this seems wrong to me. From my experience I would say most open source projects use CI, particularly because so many of the bigger platforms (Travis, Circle, CodeShip) make it free for open source.
It is an extremely valuable resource when you consider the infrastructure and integration you get with absolutely no effort. Installing more obscure libraries or cutting edge releases can be a pain, but it is minuscule compared to the time it would take to provision, secure, and manage a group of containers for an open source project.
Fully self-hosted looks like the only sane way. Harder to set up, but at least one can check the whole chain that way.