Although I love ScyllaDB and would want to see this option in the future, I don’t think this is the right time for it. It seems like most of the expertise and work being done now is around Postgre.
It’s more important to get something working than it is for it to be ideal. Because this is an open source project, there will be plenty of options to upgrade to Scylla in the near future (especially if we do a good job of encapsulating the db logic and semantics in code). Ideally, people who are running their own instances should be able to use mysql, percona, maria, postgre, scylla, or really anything they want.