Basically a good idea, both for privacy and responsiveness.
However it works generally really bad. Doesn't make difference between British and American English, it often flags correct words or makes wrong suggestions, sometimes doesn't even works and it gets stuck at half of the words.

Unfortunately, it needs a lot of work to be a useful alternative to all other high-priced grammar checkers.

It constantly flags common people names and degrees even with all the filters turned on. Just a pain to use for correspondence.

I really wanted to like it, but code starting with "harper-render-box" is being inserted directly into the text field in a forum I visit regularly. Even when I disable Harper on that particular site, a shorter string of code is still inserted. I have to edit the post to delete it. I do not know WHY it happens, only that it also seem to affect my installation of Darkreader, since some of its injected code is revealed as well. Only uninstalling the addon completely cleared the issue.

I posted the issue on GotHub as clearly as I could: githubDOTcom/Automattic/harper/issues/2913. I will try it again after several months have passed, to see if the issue is gone.

Has potential, but I can't trust it. Two times when writing a post on Reddit it decided to post by itself when I clicked a word to check the error.

Another issue is that when replacing a word, it actually paste the whole sentence I had write before, so I get two sentences, one wrong and one corrected.

Maybe it's a issue with Reddit, maybe it's a issue with Zen browser, but I can't trust an app that click in "post" for me.