In a Linux distribution for a particular architecture all code is compiled to the underlying CPU architecture. Packages can also be built from source.

Not all code is written portable. Say, many things won’t compile or won’t work under PPC64.

MIPS64 is not a very common architecture today. We know XOrg, FVWM and Emacs will probably work, so I expect this thing to be usable for many things. But not just as good as Linux on amd64.