When should you switch from a relational database to NoSQL?

hey everyone, i'm trying to figure out something for a new project and could use your input. we're creating a recommendation engine. a relational database seems like the safe bet, right, but i'm not sure if a NoSQL option, like Cassandra or MongoDB, wouldn't actually be better, given how we need to structure the data and scale things up. what makes you decide it's time to ditch SQL. what's the hardest part about switching over, in your experience