One of the few legitimate criticisms of "Indie" authors, myself included, is the uneven editing. Seriously, unless you expect to clear the $400-500 that a minimal copy-editor charges, this can rapidly run into large amounts of expense. Serious content editing can run into even higher amounts of expense.
I've tried the grammar and spelling checkers in Libre Office, Word, and the Hemingway app. In fact, I use all three on everything I release. They aren't good enough.
I had a chance to try Grammarly and can give something of an initial review.
Grammarly is a Natural Language Parser combined with a spelling/punctuation checker. As it comprehends English at a deeper level than the simple tools, it is far better than the simpler tools that come with word processors. It's not a replacement for a good editor but isn't bad.
1) It's excellent at detecting simple errors like missing articles, misplaced commas and homonymic misspellings. Things like Lent vs. leant, lead vs. led vs. lead, and they're vs. there vs. their.
2) It's decent at understanding errors like the use of an inappropriate preposition, and some verb tenses.
3) It's horrible with some things. "He'd been there." Will be flagged. "Pottage," is a medieval food and it will suggest "Cottage." These higher-level errors mean that you can't blindly accept its changes.
There is a trick that makes it easier to use. Don't bother with the online error by error report from the website. It follows the rules in the order that they were searched. Doing this means that the errors hop up and down the document. It's slow and highly disorientating.
Instead, get the pdf dump. Open that in a window with a pdf reader and open your document in another window with your favorite editor. Then you can follow the errors in the annotated pdf dump and at the same time fix them all in the original document. It is both faster and easier.