I have no problem whatsoever with Nix. I didn't say a word about Nix. You should read more carefully.
If I were intending on building a tweaked out system, I would still choose Gentoo. Gentoo is a tweaker's wet dream. But how many servers need this? Almost none, given that most servers need one or more of the following: Web, Mail, Samba/NFS, Database, various scripting languages. It does not make sense. The marginal speed improvement you will get in most server scenarios are more than negated by the hassle which Gentoo imposes on the Admin. In those cases where you DO need the bleeding edge, you almost never need it for the whole system. Need to generate -march specific code? Fine. Do it on a package-by-package basis where it makes sense. But saying, "If you need speed, you need Gentoo" is just wrong.
Blaming this on the devs vs. Portage is a meaningless distinction. When an ebuild doesn't work, what bloody difference does it make whether the devs or Portage is to blame? In the end, all it really means is that I cannot install a package. An OS is more than it's core technology. It is also the developers who build on it. To talk of the superiority of Portage when the devs are constantly breaking the dependency tree is to turn your OS into a Platonic kingdom (look it up).
And as for your note about using the help forums: I have never, not once, had a package install fail on Ubuntu. 15 server installs later, everything just plain works. In Gentoo, simple packages fail all the time.
So, that comes to the point of my original response: Gentoo/Portage is absolutely the wrong answer for a universal packaging system for all the reasons I cited. I'm sorry if you think that is my "nonsense" but I work in the real world, with real deadlines and budgets and bosses breathing down my neck. The words, "I'm sorry, sir, but the critical security upgrade for Apache didn't work last night. I found the fix on the Gentoo forums, but it will take me another few hours to rebuild all the packages that got broken" won't cut it here.