In GitHub, if you want to put an issue from a repository on a Project board it creates a project item to reference it, thus using an issue. If you have a large project (which I have where we hit the capacity of project items) then you are forced to start deleting issues from the project. Which in my opinion isn't great, because that limit includes project issues that have been marked "done". Then you lose the history of maybe why things were done a certain way.