Some packages cannot be analysed by diffoscope due to it running "forever". Currently the diffoscope process is killed after 120 minutes. https://reproducible.debian.net/index_breakages.html collects a list of currently undiffable packages (as well as other issues with tests.reproducile-builds.org.
Our notes about issues affecting packages are stored in notes.git and are targeted at packages in Debian in 'unstable/amd64' (unless they say otherwise).