My guess is that this is people who don't understand their database and have built it incorrectly for their needs. MongoDB under some circumstances is not supposed to return all records. If you want that guarantee you either need to architect your database for it or use a database that guarantees it under all conditions. This is not a reliability issue with MongoDB unless these guys produce something a lot more substantial that rumours. MongoDB is pretty good, we've had it in production at some decent scale for three and a half years and I use it in numerous places, not only with NodeBB.
If you are looking for random threads of anonymous people posting "concerns" about things that they don't understand about any database, you can find it. Everyone tied to one product likes to have "concerns" about the products that they didn't choose or don't support (or sell.) I would take this with a grain of salt. If you look at the rest of that thread, it's a joke. People who clearly don't understand data theory and use cases or schemas or relational data just ranting because they hate the idea of "new" even when it is quite old (NoSQL pre-dates relational data, I've been a NoSQL engineer since 1989 and it was OLD then - we just didn't this cool retro name for it.)
With any database, it's worth knowing if there are issues. But MongoDB is a leader and absolutely perfect for this use case. The problem with the people in that thread is that they are looking for relational systems that are not sharded and are full synchronous across shards all of the time, choose something that they didn't understand that was the wrong tool for the job and instead of admitting that they were confused and goofed on their selection they rail against the product.