I like Portage when it works. When it doesn't, I don't like portage very much.

I'm not sure if you can "alternatives" glibc in Gentoo, but the "whatever .so you have" isn't a thing there, you can slot different versions if you need to.

If I want to test some software I'll use Ubuntu with docker or whatever, but to deploy to production I will make it run on Gentoo. Hell or high water.

Windows software still ships every .dll it needs, unless it is a Microsoft one. Do a search for msvc.dll or whatever sometime and marvel at the pagination.