ffs, this attitude causes massively more problems than it solves.
I don't think that it causes so many problems to just use MySQL instead of Postgres from the very beginning of a project. I like using Postgres and I understand that I shouldn't care about scaling but if a make a good decision from the very beginning it can't hurt.
I would rather use Postgres and have a RDBMS that is quite strict and migrate data later instead of having a RDBMS that just does what it likes sometimes.
For example, query your table „picture“ with a first column „uuid“ (varchar) with the following query:
SELECT * FROM picture WHERE uuid = 123;
I don‘t know what you expect, I expect the query to fail because a number is not a string. MySQL thinks otherwise.
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.
I don't think that it causes so many problems to just use MySQL instead of Postgres from the very beginning of a project. I like using Postgres and I understand that I shouldn't care about scaling but if a make a good decision from the very beginning it can't hurt.