I really agree with your sentiments, that first paragraph is a great quote. I grew quite an adverse to MongoDB after researching it. While I never found this specific caveat, I found other very worrying decisions.
> reliable storage engine
By "reliable" I assume you mean "consistent?" While MongoDB claims that it's CP (which it's not, as per the article) there's nothing wrong with inconsistent databases (AP, e.g. CouchDB). Mathematically there is no reason for MongoDB to behave like this. It's fundamentally broken; it's neither AP nor CP.
I actually mean reliable. Its probably different now, but at launch, the defaults were fsync'ing every 30 seconds or so. It would literally just apply the change to an memory mapped buffer and just fsync it once in a while.
They did that so they could look good in benchmarks, and it's why they recommended so strongly that your memory completely fit in RAM or else things would fall apart (pro-tip, any system that recommends that has a poorly designed storage engine).
They also screwed up the consistent side of things as well.
> reliable storage engine
By "reliable" I assume you mean "consistent?" While MongoDB claims that it's CP (which it's not, as per the article) there's nothing wrong with inconsistent databases (AP, e.g. CouchDB). Mathematically there is no reason for MongoDB to behave like this. It's fundamentally broken; it's neither AP nor CP.