Google knows all about me and its assistant is, usually, great. Amazon has troves of data on what I buy, and I get to yell at Alexa to order more TP as soon as I see we're on the last roll.
Apple knows much less about me and, while I'm still an Apple fan and am tied to iPhones/Macs thanks to iMessage, Siri stinks as a result.
If voice assistants based on machine learning (specifically, personalized voice assistants) are the next big thing, Apple's privacy ethos will separate it from its major tech competitors – either in a great way, or a very negative way.