Apparently a little while ago someone at MongoDB made some dubious claims on Twitter about the database, that it “pass[es] the Jepsen test suite” and is “not losing anything”.
Unfortunately Jepsen (Aphyr) was summoned in said Twitter thread, inspiring a follow-up analysis of the database in which it was shown that its settings for read isolation don’t actually guarantee the safety they state without some additional settings on write isolation. Transactional isolation is also set differently from database and collection isolation. One might consider transactions that don’t provide the transactional isolation they claim without additional external settings to be unsafe.
It’s worth noting that misrepresenting database test results in full view of a tester that makes their living testing databases is also unsafe.
Anyway, it’s a good read.