When does splitting up a relational database become a worse idea than moving to NoSQL?

Hey everyone, I've been thinking about how we scale relational databases and I'm a bit stuck. We always talk about sharding to handle more users, but when does sharding get too complicated to manage? I mean, keeping schemas updated and dealing with queries that span multiple shards can be a nightmare. At what point does it make more sense to just switch to a NoSQL database, or maybe use a mix of both? Is there a certain amount of data, how complex your queries are, or even team size where sticking with relational just isn't worth the trouble of a distributed system? I'd love to hear your experiences and what makes you decide.