I upgraded from an S7 which works flawlessly with the voice command.
With the S7, can even hold the camera at arms length and just talk in a normal volume voice and it always worked.
With the S9, I have to shout or at least use a very raised voice. It works once and then, it doesn't work again until maybe the fifth try. Very frustrating as I often want to take a photo in an awkward position where the voice command would save the day.
Google Assistant work absolutely fine every time with no need to raise my voice, just talk in a normal speaking voice.