all config combinations aren't valid (because of dependencies), and when there isn't they are mostly independent (e.g. 2 builds with CONFIG_FOO=y/CONFIG_BAR=y and CONFIG_FOO=n/CONFIG_BAR=n cover 100% of the cases for unrelated FOO/BAR), so it's not that bad.

And folks do build stuff, including with the awesome "randconfig" mode which generates a.. random config, so while it's still possible for some specific config to not actually work at runtime it's actually quite unlikely for build itself to fail (for releases at least)