That's not really true. Oob it's configured for high availability and flexibility. But you can configure read write concerns to control how many replicas must acknowledge a change before it's considered successful if you need more consistency.
That's just the nature of a document store vs a relational db.
you can build a document db with strict serializability and you can build a relational db with the consistency model of mongo
having strict consistency does not require trading off high availability or flexibility
mongodb does not fsync by default so if it crashes it can lose “written” data
it is a dynamo-type DB, but objectively worse
