I am wondering if aurora is compatible with mysql and postgresql what is the use case for using mysql / postgresql replicas ? Why not use aurora replicas ?
Yes so you have Aurora Mysql and Aurora PostgreSQL that you can use instead and AWS will handle the replication for you for each replica you add.
If the primary instance in the cluster fails it can fail over by either creating a new primary instance or promoting an existing replica to the primary instance.
There is so much more in aurora that you could do quite easily such as using global database for regional replication, automatic scaling of storage, storage auto-repair and crash recovery.
If you are looking at using this for production, I would recommend going through the documentation. https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/CHAP_AuroraOverview.html
We changed to it to mainly reduce operation overhead and simplify the configuration of it.