Frankly, I believe that a mastery of grammar is the most basic skill (apart from correct spelling) that needs to be possessed by a would-be writer. People relying on software to do the job for them are best seeking alternative income streams that do not require writing ability.
Brad Hoover, Grammarly's chief executive is quoted as saying "Grammarly "isn't intended for professional writers to achieve the next level of language mastery, or to judge artistic prose." Grammarly is "meant to proofread mainstream text like student papers, cover letters and proposals." So it seems it is targeting an audience of semi-literate and ESL students.
In a test piece submitted by the Economist journalist, which referred to black South Africans, Grammarly objected to that term and made some disastrously incorrect suggestions, leading the author to comment: "No, none of "Stygian", "unhealthy", "despairing" and "terrible" are particularly good synonyms for "black" when you are talking about black South Africans."