While I think there is no “one shoe fits all” solution when it comes to databases in general (because each use case is slightly different and the needs are different) we still see lots of ScyllaDB users who replace multiple tools with just ScyllaDB. In this sense, ScyllaDB CAN be a one-shoe-fits-all solution if your goals are aligned with what ScyllaDB can offer.
ScyllaDB supports OLTP use cases (so no need for rdbms)
Yes it does and would be a great choice for any use cases where you require low latency, high availability, and large scale
It supports document storage (so no need for MongoDB)
You can store JSONs in ScyllaDB and you can query it. Your data model won’t be as flexible but ScyllaDB will provide better and consistent performance. We have a great user story on when to use ScyllaDB vs MongoDB
As far as comparing ScyllaDB to Cassandra, the interface is the same (CQL) but ScyllaDB is implemented with a different language (C++) and several benchmarks prove that ScyllaDB is more performant/less expensive than Cassandra.
I still might use RDBMS for some use simple cases
Yes I think that would make sense for a simple use case, and you can rely on ScyllaDB where performance and low latency matter.