pro: for a huge number of people, being on github makes it easier to contribute
con: for another (almost certainly smaller) group of people, it makes it more difficult and potentially prevents them from contributing
it seems to me that the best course of action for many projects is to be on github but to also be elsewhere, and to explicitly accept issues and PRs in multiple places. it's a mess, but it's better than either being on github or not.
fwiw, I use github in some contexts, but, because it is a giant hassle to make a new account anonymously these days, I haven't yet made one for this identity... so I sometimes irritate the lemmy devs by sending them bug reports on matrix instead of opening an issue :(