Yep, the problem with Github is that issues/PRs/the wiki are not stored in version control like the code is, as there is no standard for storing this kind of metadata. So even if you make a mirror of a Git repo, it will not include the issues and PRs as those are Github-specific metadata.

There exists no viable solution to this problem because no tool exists yet that can identify the Git hosting provider used, and export it’s metadata to a machine-readable file for importing to another provider, partly because providers offer no viable way to import this metadata.

What will probably end up happening is somebody's going to develop a set of Git plugins that do this and they will be bundled with the main Git program. Every person and business using Github already has Git so making a Git extension that’s included by default is much easier to adopt than the various stand-alone command line Github exporters (to human readable files).