It's not that nuanced in this case. These things are literally wire-taps. You can have a voice assistant that does just about as well as Alexa/Echo/etc, without having to sacrifice your data privacy.
What's your metric? And are these offline assistants smartphone apps? NLP is expensive computing, so it'd make sense for those to do poorly. And this doesn't even have to strictly be offline; You can have your data processed online in a privacy-respecting way, so long as you actually know what's going on under the hood and have control over the data.
My shill is MyCroft[0]. They're partnered with Mozilla to develop an open source NLP engine[1], so I have high hopes for it.
It does look promising, but it also seems like it has yet to get to the point of working "just about as well as Alexa/Echo/etc." – which means it's an open question whether it will be able to.
