Uber switched because of a very specific problem they had with the internals of Postgres, that was handled differently in MySQL (which I believe is now "solved"

Uber switched because of a very specific problem they had with the internals of Postgres, that was handled differently in MySQL (which I believe is now "solved" anyway). It's not that MySQL scales better than Postgres, but that Uber hit a particular specific scaling problem that they could solve by switching to MySQL. You could well use MySQL "because it scales better" and then hit a particular specific problem that would be solved by switching to Postgres.