When we added s390x to the main archive, coming from Debian Ports, we were unlucky. A new glib release had assumptions that weren't true on 64bit big endian architectures and it entered the archive just a few days before we made the initial import. This weekend we finally got a new major release into Debian unstable that fixed these issues. So we're almost on par with s390 now. It all untangled quite nicely after glib-networking was able to complete its testsuite. Only one build-dependency loop between nautilus and tracker had to be broken manually.
So what's left? There's a bunch of usertagged bugs (with both general FTBFSes and arch-specific issues; kudos to Aurélien Jarno providing a lot of patches) and we still need to file some, like iceweasel segfaulting during its build. That's important because another bunch of packages needs it to build (well, mozjs and/or xulrunner, or some package that needs those).