MongoDB is great for developers. Very facile to get started. However, it tends to fall over when it hits scale — which could be in total data set size (like, >TB scale), transaction scale (>100k ops) or in low latencies (submillisecond to single-digit millisecond).
In any of those domains, if you are trying to solve your problem with MongoDB you are in for a world of hurt.
That's generally when people start looking at other options. Whether an in-memory system for pure speed, or a horizontally scalable system for raw size or throughput.
I do use discord for a few groups, too bad they will not allow 3rd party clients because a discord terminal app similar to irssi would be awesome