Relational databases are not designed for the high-volume, high-velocity workloads that are common in microservices, and they can create a tight coupling between the microservices that use them, which can make it difficult to develop and update each microservice independently. 

And a single point of failure and difficulty in scaling each microservice independently. 

These issues can make it difficult to design, implement, and maintain a microservices application that uses relational databases. 

https://relevant.software/blog/microservices-database-management/