SQL versus NoSQL for user profile data? Its a big question, dont you think? Which one is better, really?

hey all, thinking about how we structure user profile data. for a new service, we're getting close to needing to decide on a primary database. we're leaning towards a relational model for some parts (like user authentication details, roles, etc.), but for more flexible, evolving parts of the profile (like preferences, custom fields, activity feeds), a NoSQL approach seems appealing. has anyone here gone down this path, using a hybrid or choosing one over the other for user profiles specifically? what were your biggest challenges or surprises