MySQL scales out just fine. Anyone who thinks otherwise is simply not well versed enough in how to properly shard data and manage replication topologies. I say

MySQL scales out just fine. Anyone who thinks otherwise is simply not well versed enough in how to properly shard data and manage replication topologies. I say this as someone that despises MySQL. Same obviously applies for PostgreSQL but is actually slightly harder for a number of reasons with less out of the box solutions like Vitess available.