The new aggregation operator $documents cannot be used together with $merge in a sharded cluster, you get an error:

db.aggregate([
   {
      $documents: [
         { _id: ObjectId("6616b08a610fab3e84d2d4ee"), a: 'foo', shardKey: 1 },
      ]
   },
   { $merge: { into: { db: 'myDB', coll: 'sharded_Collection' } } }
])

raises

$documents must run on mongoS, but cannot :: caused by :: $merge must run on a shard

Having this new function available in every environment would we great.