I have plastic & synthetic bone nuts as preference. Simple fact that they are easy to file & shape compared to Tusq nuts, which although good, FML when it comes to filling the slots. I've never really liked brass, since if it came to a metal nut, i'd go with yamaha's design using the same material they use for their frets which makes it sound more like a fretted note played open. crisp and clear. But this is all my preference.
I agree about the difficulty on filling any type of metal nut. But if a Brass Nut improves the sound of my instruments, it will be worth the stress on filling it. BTW i found this picture online of a Yamaha Metal Nut. Is that what your talking about?