How do you choose the right DBaaS provider for specific use cases like transactional workloads or analytics?

When picking a DBaaS provider, how do you match the platform to your use case? For example, how do you decide between relational databases like Amazon RDS for transactional workloads versus something like MongoDB Atlas for distributed databases or unstructured data? I’d love to hear how you make that decision based on performance, scaling, or specific project needs.