When emerging nvidia proprietary drivers, portage warned me that I have certain kernel configurations either set wrong, or unset, which could cause trouble for nvidia to work.
I was impressed that emerging nvidia drivers involves scanning current kernel config to see what it is missing.
My favorite part is how common packaging tasks (like checking the kernelconfig) are put into nice easy to use libraries ( eclasses) so they don’t need to be written out in every single ebuild. So for eg cmake packages you can mostly just say hey this package uses cmake, heres what options to set including some that we’re making useflags, and the cmake eclass and the defaults will just handle the rest of the build and install process.