Radicale is a very lightweight server, however, it intentionally doesn’t implement the CalDAV and CardDAV standards completely, which might lead to issues even with very well-written clients. Apart from its non-conformity with standards, there are multiple other problems with its code quality and the way it is maintained. Consider using e.g. Xandikos instead.

That said, vdirsyncer is continuously tested against the git version and the latest PyPI release of Radicale.