That reference - my initial gut feeling was that `MERGE` felt more readable, but then I read this paragraph

> If you want the generality of MERGE, you have to accept the fact that you might get unique constraint violations, when there are concurrent inserts, versus with INSERT ON CONFLICT, the way it's designed with its speculative insertions, guarantees that you either get an INSERT or an UPDATE and that is true even if there are concurrent inserts. You might want to choose INSERT ON CONFLICT if you need the guarantee.

Basically, `MERGE` is susceptible to a concurrent process also writing `INSERT` where that `INSERT` and `MERGE` are unaware of one another, causing a duplicate value to be used.