From my own experience, I have enjoyed using online grammatical aids, to a point! Here’s my humble take on them:
+ve: picks up repeated phrases, checks sentence length, and overuse of adverbs, etc. Psst, here’s where it would have helped John Grisham in “The Guardians”—on page 157, he writes, “He takes a drink and studies the ocean.” Then, on page 159, John writes, “He takes a drink and studies the ocean.” Well, software would pick that up.
-ve: does not help you develop your own writing style, nor flow or ‘voice’. There is often a cost.
Ask yourself this question: would Hemingway, et al, use an online writing tool? They might in their early days of writing, but I am convinced they would go up the wall after a while and stay their own course through the rough editing waters. Good writing and editing software are destined to call off their romance after just a few dates!