I read a blog post a few years back discussing some issues with Github's fork feature (as opposed to just creating a new repo that isn't explicitly linked to the original within Github's UI). From a quick search, I believe this[1] was it, and I remember finding it fairly compelling.
It was an interesting read because Linus has repeatedly dismissed github for kernel development, and he never explains why in detail (as a consequence most news reporting his soundbites are not very informative either)
[1]: https://zbowling.github.io/blog/2011/11/25/github/