I use transactional DDL all the time, even during development. It's nice to not have to fully reset your schema every time you test a migration file locally. With transactional DDL, you run the whole list, and if any fails, it rolls back to where you started. You look at the error, edit your migration, and try again. It really is a time saver. There is a peace of mind always knowing your schema is in a consistent state not unlike the peace that comes from a good set of unit tests.
1. Yep, I definitely miss clustering indexes in Postgres sometimes. I can sometimes fake it with covering indexes when all I want are an extra column or two along with the primary key or similar without seeking to the main table, but you're right about that MySQL/MariaDB win here.
2. The dynamic computed column is an easy workaround with immutable functions that take the record as a param.
CREATE TABLE foo ( a int, b int, c int );
CREATE FUNCTION d(entry foo) RETURNS int LANGUAGE sql IMMUTABLE AS $$
SELECT foo.a + foo.b + foo.c;
$$;
SELECT a, b, c, d(foo) FROM foo;
It's not part of the table schema when doing a SELECT *, but it is just as efficient as a computed column in MySQL/MariaDB and only slightly more verbose.
3. ON UPDATE CURRENT_TIMESTAMP works in Postgres with a trigger function, which you can reuse if all your tables use the same name for your "last_modified" column (probably a good idea anyway). Not as convenient as the declarative syntax, but it's a fairly trivial workaround.
CREATE OR REPLACE FUNCTION update_last_modified() RETURNS TRIGGER AS $$
BEGIN
NEW.last_modified = now();
RETURN NEW;
END;
$$ language 'plpgsql';
CREATE TRIGGER foo_last_modified BEFORE UPDATE ON foo
FOR EACH ROW EXECUTE PROCEDURE update_last_modified();
CREATE TRIGGER bar_last_modified BEFORE UPDATE ON bar
FOR EACH ROW EXECUTE PROCEDURE update_last_modified();
One function, many triggers. You also get to choose between "when transaction started" (now() or CURRENT_TIMESTAMP), "when statement started" (statement_timestamp()), or "right now" (clock_timestamp()).
I don't mind workarounds so much as functionality that simply cannot be replicated. For example I miss real temporal table support in Postgres like what you can find in MariaDB or MS SQL Server. The painful kludges for missing PIVOT support like in MS SQL Server is another one.
You never know how much you need deferred foreign key constraints until you don't have them anymore. Or a materialized view.