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.