While I suspect chroot are fine to build rpms, there is some case where the kernel might matter when building, such as http://www.fewbytes.com/docker-selinux-and-the-myth-of-kernel-indipendence/
So opening a bug to track that;
Having read the associated blog post, I'm going to close this issue. We tend to build on the target OS for Ubuntu, Debian, and Fedora. So this shouldn't be a problem for us. In the near term, there isn't much we can do to fix this.