Well, the Postgres docs about MERGE include a similar warning: "When MERGE is run concurrently with other commands that modify the target table, the usual transaction isolation rules apply; see [Concurrency control / Transaction isolation] for an explanation on the behavior at each isolation level. You may also wish to consider using INSERT ... ON CONFLICT as an alternative statement which offers the ability to run an UPDATE if a concurrent INSERT occurs. There are a variety of differences and restrictions between the two statement types and they are not interchangeable." https://www.postgresql.org/docs/current/sql-merge.html
That's not a warning, instead, it's how it should work, and how one would want it to work, i.e. that the transaction isolation rules apply. Lack of this, would have warranted a warning.
