It depends heavily on the project. Maybe it's using a bespoke/old build system that doesn't support targeting arm, obviously any uses of assembly need translation, bugs/compilation issues regarding signed-ness of char in C/C++, or page size. Those are at least all the issues I've encountered.
When Itanium was new, HP contracted with Progeny Linux to help them port Linux software. Despite the fact that Alpha had paved the way for 64-bit support it was a long effort.
