I really, really, really don't like to have to go to a separate system and create a new account just to be able to provide feedback or raise issues about your code. Why? Because it's a pain. Even if the GitHub issue tracking system is found to be rudimentary, the strength of it is that it's right there. There where the code is. There where the users are. There where I am and where I already posses an account. By not having issue tracking there, you are preventing me from contributing.
Ehh... What?!? Preventing you from contributing?
Well yes. If you look at creating an issue for raising concerns as contributing, then yes! Absolutely! It also shows that the project has people that cares. That it has a community. If you want me to create a separate account just for the sake of reporting something. Sorry. But it's not going to happen.
I would like to ask you a question. So from me to the organisations and people using an external system. Why are you doing this? Is it because you already have an super awesome and complex issue tracking system. With a quadruple of options and statuses? If you do, you are probably a commercial company, potentially earning money around an echo system where parts of it is represented on GitHub. If this is the case, you could probably afford setting up a bridge between GitHub and your other issue tracker and therefore letting all people easily contribute. Maybe a crazy idea. But despite how crazy it may seem, I'm sure there is more people like me. And maybe one off us has something vital to contribute with. This vital piece is something that you will miss. Why? Because you are only a semi GitHub peer.