I remember scirate is pretty old - the domain was registered in 2007. Given the low number of comments now, and with those are concentrated into a narrow part of arXiv, it's unlikely to take off generally.
Of the four, scirate.com seems the best option (unless you're focused on machine learning -- then arxiv-sanity is the winner).
I agree that comments on arxiv papers are "unlikely to take off" in either a commercial money-making sense or a "grow as large and popular as HackerNews" sense.
[0] https://graphthinking.blogspot.com/2021/12/comments-on-arxiv...