Specifying a field selector that contains fields within arrays when calling the mongoexport utility has unexpected behaviour, at least when exporting to CSV.
E.g. You have the following document in your db:

{"id_": "username1",
"telephones":
[

{"type": "mobile", "value":101010}

,

{"type": "landline", "value": 202020}

]
}

Running the following doesn't return the expected results:

mongoexport --csv -f '_id,telephones[0].value,telephones[1].value'