I agree that the majority of UTAUs do suffer from an unnatural sound, especially CV voice banks.
But there are a few gems out there that can sound really good. (The ones that come to mind are KaiKim, Toby Osbourne, Hachimitsu Sake (蜂蜜サケ) and Galvan Ize.
Although it requires work on the side of the UST/VSQ maker to fully utilise the voice.