The issue is that systemd might change that interface at any time, even in a minor or patch release. Because systemd's released as a single monolithic project, they don't have to commit to any standardization, deprecation cycle or the like for the interface between components.
Yep, no stable interface for journald. Someone is gonna have to prove to them it is needed, by implementing something useful that utilizes said interface.
but as documented previously in thread, for plenty of their components they have committed to stable, documented interfaces.
