Concurrent deploys have been a problem for us when using workflows. E.g. I merge commits A and B in rapid succession and two deploy workflows run at the same time potentially failing if my deploy task can't handle concurrent runs or deploying an older revision than expected when the builds finish out of order.
A partial, and possibly easy to implement, solution to this would be if we had access to a build number shared across all jobs in the workflow so we could at least identify build order (currently CIRCLE_BUILD_NUM is set when a job starts so the deploy job for the workflow triggered by commit A might be higher than the number for the deploy job in the workflow triggered by commit B if any of A's preceding jobs are slow).
This would also allow us to easily tag releases with a workflow number which might prove to be more human friendly than a job number because it would increment less dramatically (we run ~12 jobs per workflow iteration).