The documentation for $lookup says:

To return all documents, specify an empty pipeline: []

But he following two pipelines have different plans:

db.part.aggregate([{ $lookup: { from: 'partsupp', localField: 'p_partkey', foreignField: 'ps_partkey', pipeline: [], as: 'partsupp' } }] );
db.part.aggregate([{ $lookup: { from: 'partsupp', localField: 'p_partkey', foreignField: 'ps_partkey',               as: 'partsupp' } }, { $unwind: '$partsupp' }] );

The plan where the pipeline argument is not present at all is a full-blown SBE join pipeline. The plan where the pipeline argument is an empty array is a single-line SBE table scan followed by a classic $lookup plan.

My understanding is that the two queries are equivalent and therefore should produce equivalent plans.