I appreciate the time you took to install and test all these distros and then write this article. I'm going to give NixOs and GuixSD a look.
But I can't help but wonder why one would run Gentoo or Funtoo, except on a personal machine when you have a lot of free time on your hands.
The merits of compiling everthing from source for hours, probably days to gain a few percentages of performance seems pointless. And when an upgrade arrives, you get to wait for the compilation to finish.
I agree that using custom USE, C(XX), MAKE, ... flags and options sounds very l33t and cool, but when they cause unstable or unusable binaries, the upstream devs will probably not be too eager to help you out.
Yeah I agree with your opinion on Gentoo/Funtoo actually. It's why I don't use them everywhere. But I do find them both to be very convenient for programming. For example, if you need a cross compiler for another architecture/OS, there is no distro that makes it easier than Gentoo. Also having all the configuration is just something that comes in handy every once in a while, and saves me the effort of having to compile libraries from scratch myself (which I end up doing a ton on other distros). That said, Gentoo/Funtoo are not something I'd want to use in a production environment because updating anything would mean hours of downtime.