Expressivity and naturalness are still not good enough. It's getting there, but today you'd never have a voice conversation with an AI and come away thinking "boy, what an engaging, charismatic personality that AI had!" This is partly due to latency, partly due to imperfect speech-to-text which is too slow and doesn't understand conversational dynamics or nonverbal signals, but yes partly due to text-to-speech.

When it comes to naturalness for dialog, OpenAI actually has the best voices I've heard—significantly better than Azure or 11 Labs which are second and third best. But at least the voices they expose through ChatGPT are fairly muted and inexpressive.