The current warning reads:

• • WARNING: This replica set is running without journaling enabled but the
  • writeConcernMajorityJournalDefault option to the replica set config
  • is set to true. The writeConcernMajorityJournalDefault
  • option to the replica set config must be set to false
  • or w:majority write concerns will never complete.

While the consequence mentioned is true, we should also indicate that, regardless of w:majority writes, memory usage on both mmapv1 and WiredTiger will increase to the point of eventual node failure.