unar is unable to unpack certain rar files, like: https://mega.nz/#!TZUGyKBI!5rITrOXUUBXaThpWfwxc_Hks-UI64eKi7O_336Vn4og
unar for other systems manages to unpack this file. The reason is one of the patches that our upstream (Debian) applies:
fix-crashes.patch CSInputBuffer-bit-string-reading.patch gnustep-support.patch proper-init-autoreleasepool.patch
Other patches applied:
document-undocumented-options.patch use-system-wavpack.patch pass-cppflags-for-fortify-functions.patch remove-build-date.patch
do not influence this issue.
I suspect that the most likely cause is the fix-crashes.patch patch, but I couldn't confirm this because the 4 patches need to be applied all together, else build or patching fails.
I suggest leaving out the first 4 patches (fix-crashes.patch, CSInputBuffer-bit-string-reading.patch, gnustep-support.patch and proper-init-autoreleasepool.patch) and to leave whatever fixes they do to the main developer of unar to implement.