Mongo is the wrong choice for a solid 75% of the places it's used. In the vast majority of cases, it was brought in to replace a relational db because developers though it would be faster to not have a schema / constraints / relationships, etc. It usually lets you develop faster, at the cost of blowing up in your face a few months/years down the line, when you have to rebuild your app to use a real database because your devs remembered why relational dbs are useful in the first place.

Mongo is a document store, not a relational db. Mongo is a good choice if you're looking to replace ElasticSearch, not if you're looking to replace MySQL.

Nowadays not much, but it used to be overrated and had serious reliability problems.

Your startup probably doesn't need Big Data (TM). Just use a relational database like Postgres and learn a bit of SQL. IIRC, Postgres outperformed Mongo at JSON processing, which was supposed to be one of the stronger points of MongoDB.