When working with SFDX, we hit the wall using
sfdx force:data:tree:export which failed when we had lookups and master-detail relationships. We had to resort to writing some custom code to ensure we would write the parent, then the child, then update the lookup on the parent object towards the child.
For other use cases, we extracted the data simply via a set of SOQL query via REST API: about ten custom objects, each with around ten fields. Can the result of those queries be piped to another rest API, on a different org, to re-import the data? In this case we do not have a lookup, but we are not able to write a single SOQL query to be used with the