When using the WorkspaceRunner you often want to do something after that transformer with the results of the child processes that it spawned. Sometimes this involves launching another Workspace. At the moment there is no easy way to understand if all the child processes from that WorkspaceRunner have been fully completed.
This is the crux of the problem, from the help:
If the Wait for Job to Complete parameter is set to No, the initiating feature is output through this port if the request was successfully submitted, though whether or not the workspace completes is unknown in this case.
One option might be a new port that releases a feature when the child processes have all been closed off, or some new parameter/mode. At the moment you need a workaround or you need to throttle the process and use 'Wait for job to complete'.