That is true, but I wanted to share another angle that may or may not affect PostgreSQL while it continues to affect MySQL even as it has crash-safe (though not transactional) DDL now: these schema changes are online for the master, but are not replication-aware and can have impact in replication delay on servers down the hierarchy.

For this reason alone I think we'll continue to use schema-change tools on MySQL even if the server itself becomes better at those.

In the specific case of gh-ost, another good point is that migrations can be completely paused, which in MySQL is not true of online DDL.