Currently we only push down $match & $project stages across $unionWith branches. However, any stage that operates on a single document at a time can safely be pushed down. For example:

{$unionWith: {coll: ..., pipeline: [...]}}, {$lookup: {...}}, {$unwind: ...}

Is equivalent to:

{$lookup: {...}}, {$unwind: ...}, {$unionWith: {coll: ..., pipeline: [{$lookup: {...}}, {$unwind: ...}]}}

This is useful, because the first version was not eligible for SBE pushdown, but the second version is.