This is what MongoDB says about journaling:
To provide durability in the event of a failure, MongoDB uses write ahead logging to on-disk journal files.
But with WiredTiger:
In between write operations, while the journal records remain in the WiredTiger buffers, updates can be lost following a hard shutdown of mongod.
If journaling in WiredTiger can't fully guarantee that there will not be lost updates we have to write our application to be aware that some documents may just "disappear", so why enable journaling at all, considering that it will slow down writes?
It seems that if you can't lose updates* you have to choose MMAPv1 with journaling and if you can you may prefer to choose WiredTiger without journaling.