When trying to install a (non-widely-popular) package for the first time, if it starts building, someone can easily think that this is simply because it isn’t cached by Hydra. Then you wait a few minutes / hours, only to find that the build (or the tests) are broken and the time spent on your machine was just wasted. When you start digging in the nixpkgs repository / Hydra, you discover that this package was supposed to build on Hydra, it’s just that it was broken there as well, and thus no binary was available.
It would be nice to get a warning and a confirmation prompt before a package starts building when a build of the same derivation was attempted on Hydra but failed eventually.
Another solution would be to refuse advancing a channel if there are undeclared broken packages. Then a bot could add the broken status automatically if adding it by hand is too much work (although it shouldn’t: maintainers are usually aware of broken packages, so they should either immediately fix them, or mark them as broken).