Haven't used Grammarly in years; however, I can tell you a couple of cons Harper has, compared to Grammarly:
1: It is English-only, for now, although it has all common English dialects
2: It sometimes gets pronunciations of abbreviations wrong, although this is getting less and less common, since the developers work a lot on it
3: There aren't plugins for it on certain editors (e.g. Notepad++), although it has for Chromium/Firefox, VS Code, among others
4: It does lack many style suggestions, but I shall reiterate that they're working on it
NPM should be destroyed, at this point… so many problems come from that place.