When brainstorming the design for $unionWith, we are concerned about an issue we have previously run into in such a scenario. esha.maharishi sums up a possible deadlock that can occur when a shard tries to look at the CatalogCache while still holding a lock in this comment. It's also described in SERVER-37398 where we added this invariant to ensure this never happens. This was discovered when working with improved $out (now branded as $merge), but it has implications for sharded $lookup and now $unionWith.

This ticket tracks a feature request to remove that invariant. It should be possible to query a different shard using the CatalogCache while you are in a transaction (and thus hold some locks which you cannot drop). Currently you cannot do this because it may deadlock.