I have not used this but it caught my eye. Biggest downside is needing a github account to comment. Really narrows down the audience. But I did not know a comment system existed that is both free, ad-free, and doesn't require you to self-host (which of course wouldn't really be free anyways). Please share more along these lines if you know of them. Even just self-hosted open source. I'm also curious what comment systems people like in general.
Using github issues seems clever and hacky. I wonder if there are any major 'gotchas' with co-opting gh issues for this purpose.
edit: Similarly, there is giscus. using github discussions. https://giscus.app/