The code works for me. One thing you may want to watch our for is the way you configure your Destination Rule. If you do not use the Plan Per Destination option for instance (with the "Resolve all roles and groups to users" option enable), it will only create one Worklist item for each Role used.

Easiest way to verify your results is to use the Worklists option in Management Console --> Workflow Server, that should correlate to what your API call returns.

In our case, the three worklist items are for  tasks from three parallel branches of the workflow, not three destination users of the same task.  Sorry, I should have made that clear in my original post.