When using a Live Model data source, Pulse Alerts UI changes to reflect this, showing an option for the alert frequency, which only appears when the data source is a Live Model.
For a dashboard subscription, the UI does not change with the option for "Every ElastiCube update" being present and seeming to be selectable, though it is not. Please hide this option when the primary data source on a dashboard is a Live Model. Optionally, you could look into hiding only when all widgets on the dashboard use a Live data source and none use an EC data source.