As a consequence of privilege delegation in MongoDB, a MongoClient must eagerly authenticate all credentials that it has been provided. It does not know whether a user has been granted a privilege in another database (e.g. a user defined in database "db1" may have been granted a privilege to read from database "db2"). This makes it dangerous in general to create a MongoClient with more than one credential.

In addition, the session support added in MongoDB 3.6 requires that only a single user is authenticated.