> Postgres already has the `Insert foo on conflict do update` type syntax which I think is generally better.

INSERT... ON CONFLICT is awesome, but it has some limitations.

The one I ran into most commonly is that it can only handle exactly one unique constraint on the target table. So if you have both a PK and another unique index, you need to choose which one gets the simple 'on conflict' and which one gets a hacky workaround (locks/transactions, triggers, exception handling, etc.)

If I'm reading the MERGE docs right, you can handle that case:

   WHEN MATCHED AND old.pkey = new.pkey THEN UPDATE SET value = new.value
   WHEN MATCHED AND old.col1 = new.col1 AND old.col2 = new.col2 THEN UPDATE SET reps = reps + 1
   WHEN NOT MATCHED THEN INSERT [...]

MERGE does not have locking built in in the way that ON CONFLICT does, so it handles more cases and fewer cases all at the same time.

Agreed, another vote here from painful experience to avoid MERGE in SQL Server like the plague.