Gentoo's approach of "don't even mark your ebuild ~arch unless you've tried it on a Gentoo system of that architecture" makes that last problem very likely. Debian and derivatives take the opposite approach: try to build it for everything unless somebody says otherwise.
Some software is non-portable, sure, and yes you do run across the odd thing that fails at runtime on certain architectures, but it's not quite as bad as all that. I'm much happier to defend Debian's choice of sides in this tradeoff than I would be to defend Gentoo's, which seems to involve an awful lot of manual wheel-spinning.
(Of course, maybe I'm wrong about how much is manual. My only substantive experience of Gentoo development is getting a libpipeline ebuild into the portage tree - four weeks later nobody's turned it on for anything other than ~x86 and ~amd64. Maybe I should be doing something but it's not clear what's best ...)