Why no REST API data source?
Like to query data from SaaS apps? e.g. Hubspot? The rate limits on most API's like that normally prevent analytical queries. e.g. if you wanted to query all your open deals in Hubspot, it could take several minutes to run the API requests to get the data. This is normally solved with ETL into a database instead.
The ETL approach makes sense but in the case of a service/microservice but I'm assuming you've got to have a well defined dataflow.
Edit: also at that point, any change (schema change, etc.) to the underlying data store represents a change to your dashboard, rather than a change to an API.