Current Voice assistants aren't much more than hardcoded GOFAI software. The only relevant ML involved is in the speech-to-text model. Modern language models are on a completely different level. Problem is that they need absurd amounts of VRAM, so running them locally on a phone is out of the question.

> It isn’t at all clear what these interfaces can and cannot do, all you can do is keep trying different things until you get a result.

You would ask an LLM based assistant what it can do, in the same way you can ask ChatGPT what it can do.