why would you create separate site and not subreddit where you can get way more traction from existing users?..
But one huge perk of not having a subreddit is you don't have the mass of users come from other subreddits where the standards of comments tend to be more copypasta troll-like than HN.
I somehow didn't notice this on relevant subs (machinelearning, singularity, etc)
There was something similar a few years back, upvote.pub: https://news.ycombinator.com/item?id=16273171
I don't know the reasons for it folding, but I imagine it was difficult to sustain.