Separate Failed and Terminated in Reports
The workflow instances report includes a column for "unsuccessful instances". This includes workflows that that failed or were terminated. This information should be broken out into two different values since there is a workflow action to terminate the workflow. If you have this in a workflow and it runs how it is supposed to, I would not consider that "unsuccessful" but successful as it ran as it should. If the workflow is manually terminated, which often happens when it gets suspended for some reason, I would consider that unsuccessful.