MongoDB's biggest weakness was its default settings, which made it insecure and very vulnerable to data consistency failures. An absolute nightmare for PR. It's not perfect, but it's comparable to other NoSQL databases now.

Their biggest weakness was that their marketing promised way, way more than they could deliver.

Aphyr's original examination [0] took them to task so much so that I always think of it as the start of the "end", at least of the "web scale" obsession.

[0] https://aphyr.com/posts/284-jepsen-mongodb