I think it depends on the Discord -- if you have a group that are all on board with it, you can get some good asynchronous/slow-paced discussions ticking along in a group chat, I'd think -- but you're right that it doesn't lend itself as well to either long comments or to commenting on an older conversation without full-on reviving it into the current main topic.

(And, selfishly, I find DW more congenial and also have a much easier time replying to comments by email during the workday than I do doing anything by Discord during the workday, when I have to type on my phone to say anything. But mostly it's that this seems more fun than a Discord full of primarily strangers!)