Isn't contradictory that MongoDB may lose updates with journaling in WiredTiger?
It doesn't matter if it's a wiredTiger or MMAPv1, they are just storage engines. Whether you lose updates or not depends on write concerns.

Your app communicates with MongoDB server that writes mostly to memory that has a cache of pages which is periodically written and read from the persistent disk. There is also a journal that logs everything.

When your write to the database it writes both to pages and journal simultaneously. Pages will write to the disk depending on memory pressure. By default you don't wait for acknowledgement from the journal because the journal may not write to the disk for a while, which is represented by j=false. And by default w=1 which means it only acknowledges write to the cache pages. 

Default: w = 1, j = false - this is fast, but if something happens before the journal has a chance to write to the disk then the data is lost. What's worse is that it will show as if the data has been written/persisted if you rely on w = 1.

w = 1, j = true - in this case you wait for the acknowledgement from the journal that it has written to the disk. This is a lot slower, but ensures the data is persisted.