Asynchronous systems fundamentally differ from completion-based systems in that the responsibility for coordination lies with the caller rather than being managed internally. In completion-based systems, tasks can execute asynchronously with internal synchronization, allowing for a seamless user experience. In contrast, the async systems require the caller to actively manage the workflow, which can complicate task management and understanding for developers. This shift in structure highlights that certain design patterns familiar in completion-based systems are no longer feasible in an async context.