> In fact, nobody guarantees that say Fedora will run on an Ubuntu-built kernel.

"nobody guarantees" just means that you can't externalize the work of trying it and seeing if it works. I don't think that's a huge loss, considering the space of all possible kernels, configuration switches, patches and distro packages is huge.

It's like refusing to use a hammer because nobody can assure you that hammer A was thoroughly tested with nail type B.