Harper is so basic that I can't recommend it.

Does not catch a singular/plural discrepancy between the subject and the verb in a sentence--a common mistake when the expressed thought applies equally to one thing or to many things.

Does not catch a missed indefinite article--a common mistake for speakers of languages that don't have articles. Similarly, does not catch the use of the indefinite article for a thing already mentioned before.

Does not even catch the obvious "don;t" typo.

Have you raised these as issues? The developer has been extremely responsive in my experience.

I use Harper in my Neovim setup and its been great. I just add things that are missing from the dictionary when I come across them in my codebase

Open ticket about "don;t" typo, see https://github.com/Automattic/harper/issues/1559

That's mine! And it's already fixed; I am indeed amazed by the developer's responsiveness.