It's fair to distrust something because you were burned by using it in the past. However, both the examples you named -- Postgres and FoundationDB -- have had similar concurrency and/or data loss bugs. I have personally seen FoundationDB lose a committed write. Writing databases is hard and it's easy to buy into marketing hype around safety.
I think you should reconsider your last paragraph. MongoDB has a massive community, and many large companies opt to use it for new applications every day. Many more people want to use that product than FoundationDB.
Can you elaborate on why ‘many large companies’ are choosing MongoDB over alternatives and what their use cases are? I’ve been using Mdb for a decade and with how rich the DB landscape is for optimising particular workloads I just don’t see what the value proposition is for Mdb is compared to most of them. I certainly wouldn’t use it for any data intensive application when there’s other fantastic OLAP dbs, nor some battle hardened distributed nodes use case, so that leaves a ‘general purpose db with very specific queries and limited indexes’. But then why not just use as PG as others say?
I think you should reconsider your last paragraph. MongoDB has a massive community, and many large companies opt to use it for new applications every day. Many more people want to use that product than FoundationDB.