Currently when deciding on a backing store for my python based applications I usually jump straight to PostgreSQL what are the things that Mongo excels at these days that are missing from pg?
Saying that Mongo is the choice just for scaling is silly.
That said, Postgres is my go-to database as well, almost always, and I've never much needed anything different. Yet. Waiting to find something that it doesn't fit.
I usually just use JSONB columns for this.
When you work with Mongo in C#, you’re dealing with strongly type collections where the compiler enforces consistency in your schema and your strongly typed linq queries are translated to MongoQuery.
I would not use a schemaless data store with a weakly typed language.
I wouldn't say Mongo excels at these, but basic distributed stuff, like failover, sharding, rebalancing, resyncing, operations in general, etc. PostgreSQL can never get to MongoDB's level there, not even theoretically.
Start everything with PG.