I have a site collection, where external users are allowed to fill in a form which triggers a SharePoint 2013 workflow upon saving the list item. These workflow fails with a status “Starting” with http unauthorized. This does not happen on the workflows that based of SharePoint 2010 workflow engine. This is happening due to the dependency of the user profile for the workflow manager. Sohel's Blog: SharePoint 2013: Workflow Manager Installation and Configuration
The user profile creation in our O365 take 5-6 minutes from the time the user accepts the invitation and some of our customer sites it takes more time due to the AD Sync.