Siri Shortcuts does now work after migrating to the S4 (which naturally entailed repairing).
Initial thoughts are that Siri Shortcuts are less capable than Workflow was. For example, there is no Siri Shortcuts Watch app, and one of the shortcuts I use most calculates fuel economy so needs you to enter miles and litres when you refuel. I used to be able to do this right in the Watch, but now it tells me I need to use the Shortcut on my iPhone. 😵