In this context "broken" means "will fail to work under various circumstances in which you would expect it to work" rather than "will fail under all circumstances". The work required to make separate /usr work in all circumstances would involve moving a large part of /usr to /, which kind of defeats the point.
The fundamental problem with a separate /usr is that writing a tool that may be used in the booting of some estoric platform or setup would require you to limit yourself to the libraries available in /. Those vary across distributions, so which subset do you use? If you want to use a library that's in /usr, do you duplicate the functionality in your code or get every distribution to move it to /? What if that library has dependencies on a pile of other libraries that are currently in /usr as well? The / and /usr divide artificially split the platform in two, without any clear semantics as to which subset of functionality you can actually depend on. The world has got more complicated than it was when the worst case was /usr being on an NFS server connected by ethernet. Something has to change.