I'm using Amazon Polly for a few of months to make videos for language learners. And I realize English voices powered by WaveNet slightly better than those of Amazon but the default Japanese sounds way too worse. Anyway, their pricing and platform are almost same with Amazon, so I definitely need to add another interface for this TTS into my app. You can listen to Amazon Polly voices with the video I made: https://www.youtube.com/watch?v=ysMp0k4oR5c