When I use CallbackURL() in a script, if the app does not return the x-success value, the script will hang - I presume until a timeout value happens.
For example, if I call an app that has a “Return to Drafts” button, but instead Cmd-Tab back to Drafts, the action step is stuck awaiting the return. Another example: when I link to Working Copy and my action throws an error, like “Invalid URL key”, my action is unusable after that. I have to quit Drafts and relaunch to use the action again.
The URL step does not have this issue.