waiting for upset improvement.
Getting MERGE right is one of these "pick two" things. You get to chose between "fast", "doesn't corrupt data" and "doesn't throw errors at times" but you only get to pick two.
upsert has been an embarrassment for Postgres for far too long now. Particularly obnoxious was the "upsert is hard! Roll your own" stance taken.
However, it's not so much a performance question as one of concurrency. The easiest way to get the right semantics is by locking the entire destination table against concurrent writes. Maybe that's fine for a lot of users. But it has caused some hesitation, because other users might be disappointed.
It would be fair to say "just do it" though.