Some come to async from callbacks and others from (green)threads.

If you come from callbacks it is (almost) purely an upgrade, from threads is it more mixed.

It is a tool. Some tools make you more productive after you have learned how to use them.

I find it interesting how in software, I repeatedly hear people saying "I should not have to learn, it should all be intuitive". In every other field, it is a given that experts are experts because they learned first.

Frankly, async being non-intuitive does not imply that manual concurrency handling is less so; both are a PITA to do correctly.

It is. A lot.

But concurrency is hard and there's so much you syntax can do about it.

It IS intuitive.

After you’ve learned the paradigm and bedded it down with practice.