Ubuntu also inherits quite a bit of 's390x' platform compatibility work from Debian. For a user coming from a non-mainframe Linux background who wants the normal packages to "just work", it might be further along than other Linux distros (but: I have not quantified this). Debian-on-Z doesn't have as much testing as x86, of course, but since it's one of the five officially supported Debian architectures (x86, ARM, PPC, MIPS, S390x), it gets constant autobuilds of the whole archive, a reasonable amount of debugging effort, and release-engineering attention.