While you are correct that 3.4 now passes, I feel like your interpretation here is a bit optimistic. It's not like I tested Mongo and it passed out of the box--it failed Jepsen tests, and not just with a read anomaly--it lost majority-acknowledged inserts. The v0 protocol still fails--it's fundamentally broken. The v1 bugs are fixed now, but that's a consequence of our collaboration.

I'd say the marker of maturity here is that MongoDB has put significant time and effort into correctness: they take clock skew and network partitions as serious failure modes, they've redesigned their replication protocol, added options for stronger reads, and invested in their own correctness test suite, and Jepsen tests, as a part of their CI process.