Functional is a scale, not a binary. You could well say that Nazi society was really functional and went amazingly. However, it was not-in-small part to their actions that their society was defeated. Afterwards, it was reformed in a way that has not only done far better, but has lead to more people being happy, productive, and living decent lives. Even a society that looks like it is functioning, can be dysfunctional. A society where any group of people is pushed down and kept from their rights to live is not a fully functional one.
I have whiplash. Could you please let me know if a society can be functional and hate people, or not? Are you now defining "functional" as in "achieved the aims of that society" - because the Nazis didn't quite manage that. They did somewhat lose that big ol' war. Are you defining "functional" on a scale as in "compared to how the Japanese treated the Chinese during the same war, Nazis look god damn friendly!"? A society where one group despises and attacks another for ANY reason, is not a functional one.
You could well say that Nazi society was really functional and went amazingly.
I am defining functional in the common sense way of "doing well". A society can do perfectly well while having issues. That doesn't mean those issues can't be worked towards being fixed. You can look at the Nazi regime and say they did X and X and X. Which is true, but it doesn't mean that the whole holocaust thing didn't hurt their society immensely.