Support for online schema changes has been getting a lot better in recent releases of MySQL. No idea if it is totally bulletproof right now though, we are still in the "stone ages" of doing failover based schema changes, not even pt-online-schema-change.

It seems that the replication arguments in the post are pretty weak (migrating to 9.4+ to allow heterogeneous versions would still have less downtime than migrating to MySQL), but clustered indexes seem like a huge win for their use case of updating only a subset of each row. Whether that is reason enough to totally ditch postgres, I don't know.