I tried this after LanguageTool went subscription model. Is it ironic that the creator talks about the competitions shortcomings but does nothing to address those that occur in their own checker.

First off it's only fast when it's wrong, but that doesn't mean it will always be correct when it's slow.
One of my biggest peeves is when i spot an error myself and it hasn't, so I grab the mouse to move the cursor to do the edit and at the last moment it underlines the error as I place the cursor, which freezes the cursor and I can't type anything. In order to release it I have to accept the suggested correction or click on dismiss. If the suggestion isn't right and I dismiss it, then cursor is no longer within the text that needs editing.

It often questions my use of "to" instead of "too" in places it doesn't make sense.

It warns me of an unpaired quotation mark before I've written the second word after the opening quotation mark.
In the first 2 weeks I bothered with sending a report with each new error I encountered but when they continued to occur I found it easier to remove it's access to the sites that comment a lot on.
It identifies spelling errors but leaves half of the word there when it replaces it. So something like "confesion" will end up as "conconfession". Sometimes it will remove some of the next word instead. Then it marks that as an error but can't offer the right correction.
I have perservered with it for about 6 weeks but I am done with it now. The straw that broke the camels back? Telling me my sentence is too long while underlining the full stop - space - capital letter sequence within the "sentence" that's too long. It never offers a suggestion to remedy it and nothing I do to alter that passage of text satisfies it.

Please improve it. Also, I use English and German at the same time. Right now this makes it inpractical.

eh, not much better than the built in one. what REALLY REALLY drives me crazy and makes it totally unusable for me, is the fact that you cant click on ANYTHING that it has a line under it, whether its a green line or a red line, if you try to left click to put your cursor in ANYTHING that has ANY line under it, itll just pop up that menu as opposed to putting your cursor there. that gets SUPER SUPER SUPER frustrating after a while because if i want to change a word thats in the middle of a sentence that is underlined with a green line, well i have to click the cursor some where outside of the stuff thats underlined, and then use the arrow keys to manually go over to the word, ridiculous.

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.