• $unionWith is not allowed to read from a sharded collection if it is part of the inner pipeline of a $lookup.
• Disallowed within an update pipeline
• Disallowed alongside a $changeStream

The sub-pipeline has the following restrictions:

• Cannot contain stages which write data ($merge, $out).
• Cannot contain a $changeStream stage, or any collection-less aggregation stage.
• Cannot contain a stage which must run on mongos or the local host that the command was sent. At the moment, the only relevant stages are $listCachedAndActiveUsers, $listLocalSessions, $currentOp with localOps: true, and several internal change stream stages.