As per SERVER-7926, using MapReduce output could end into a serious data consistency issues.
The documentation must warn and push users away from using MapReduce on sharded clusters.
Investigate changes in SERVER-42134: Deprecate mapparams, out.sharded, and out.nonAtomic arguments to mapReduce
Map Reduce with sharded output can apply reduce on duplicate documents if a migration happened
Deprecate creation of new sharded output collection in mapReduce