Datomic scales horizontally for reads but doesn't for writes, but then natively mysql doesn't scale for reads or writes, most people looking to scale mysql up use a clustered approach with many readers and one writer using a third party or enterprise solution
I don't know of a reliable multi write system for mysql that doesn't make significant trade offs
At work we use multiple mysql servers to handle "scaling" so it's not a surprise to me that Nubank are using multiple Datomic servers
Unless you start getting into eventual consistency territory I don't think distributed writes are a trivial problem and targeting Datomic for this is a bit odd
Also the spec stuff I don't think it's a surprise to Nubank that spec is being upgraded it literally says alpha in the namespace, they have some great Clojure engineers at Nubank particularly the developer of Pathom they're really pushing the envelope in terms of distributed graphs and front end
I don't know of a reliable multi write system for mysql that doesn't make significant trade offs
At work we use multiple mysql servers to handle "scaling" so it's not a surprise to me that Nubank are using multiple Datomic servers
Unless you start getting into eventual consistency territory I don't think distributed writes are a trivial problem and targeting Datomic for this is a bit odd