when should you switch from relational databases to NoSQL?

hey everyone, been thinking a lot about data storage lately. we're mostly on postgresql right now, and it's been great for our core services. but as we scale and deal with more varied data types (user profiles with flexible attributes, activity feeds, etc.), i'm wondering when the right time to consider migrating parts of our system to a nosql database like mongodb or cassandra is. what are your experiences? did you face similar growing pains? what was the tipping point for you