Also TBH now that I don't have commercial reasons to push interop, if I write another document database on top of FDB, I doubt I'd make it Mongo compatible. That API is gnarly.
Out of the many MongoDB criticisms, this one is valid. This https://www.linkedin.com/pulse/mongodb-frankenstein-monster-... article is quite right about it (note: endorsing the article does not mean I endorse its author by far).
Other than that, they totally did a fake it until you make it with MongoDB 3.4 passing Jepsen a year ago and MongoDB BI 2.0 containing their own SQL engine instead of wrapping PostgreSQL.
(I've tried to keep the above as dry as possible to avoid dragging the arguments around this situation into this thread - and I suspect the phrasing of the previous comment was also intended to try and avoid that, so let's see if we can keep it that way, please)
The Will Wilson I remember was not prone to such understatement. That API (and the corresponding semantics) was a nightmare.