I used to work in a managed investment department of a large investing company in midwest. We used MongoDB as a single source of truth for daily market analysis data. We used C# .NET Core. I loved working with mongo, although it gets slow overtime but it was enjoyable. One thing I really hated was the C# driver. I just hated it. It fails translating complex LINQ expressions into mongo query and its BSON implementation is extremely slow. We literally wrote the data-access-layer in python wrapped it as an API and we got a 10x speed boost. Eventually, we replaced it with Postgres and utilized its JSON columns, and retired MongoDB. Good memories learning its query syntax and investigating where the slowness was coming from.