Big problem with GitHub issues is how simplistic those are. All
categorization is done by labels with no internal
differentiation, advanced search queries are not possible. Also
bugzilla is self-hosted solution we have full control about and
with GitHub you can only rely on what external API exposes.
Also consider effort needed for moving all issues from existing
database and breaking routine workflow of all active dmd/Phobos
developers.
Personally I don't think this is as much of an issue. Issues
don't get much attention because there are only few people
looking at them (other than own issues). Most D users don't have
any reason to do so.
