Long ago, I probably shot close to 1000 weddings. I gave up on automatic flash and never looked back. Once you get the hang of it, it becomes very easy to manually adjust flash output and estimate the required aperture for ALL conditions. It just takes practice and understanding. For super critical use, you can simulate a scene in advance and test using an incident flash meter. Once you get this method in your mind, you will never go back to automatically measuring any sort of reflected flash.
You need to measure the intensity of the light to get a correct exposure. Its reflection is dependent on too many other factors - as explained above.
Nevertheless, I'll bet almost everyone will disagree with my manual approach and incedent meter reading. To each his own. I prefer doing the math and knowning whats happening.