With non-default settings[0].

The Jepsen tests passed with the "linearizable" read concern. The default read concern is "local", which "Provides no guarantee that the data has been written to a majority of the replica set members (i.e. may be rolled back)."[1] This is like having "READ UNCOMMITTED" be the default read level in a traditional database system.

The Jepsen tests passed with the "majority" write concern. The default write concern is "1", which means only the "primary" in a replica set needs to acknowledge the write[2]. This does not guarantee safety in the face of network partitions.

It's still not safe out of the box.

[0] - https://jepsen.io/analyses/mongodb-3-4-0-rc3

"With the v1 protocol, majority writes, and linearizable reads, MongoDB 3.4.1 (and the current development release, 3.5.1) pass all MongoDB Jepsen tests:"

[1] - https://docs.mongodb.com/manual/reference/read-concern/

[2] - https://docs.mongodb.com/manual/reference/write-concern/