For example: I wanted my personal assistant to track hygiene, which is a natural use case. But then you arrive at the natural conclusion that either a) the user needs to enter the data themselves (“I brushed my teeth and washed my face and took X medications at Y time”), or b) you need some sort of sensor in the bathroom, ranging from mics or radio sensors up to a tasteful camera. And a million subtle versions of (b) is where I see people going “no, that’s weird, it’s too much info all together”
Some data might never travel across a Google account, but very well over ChatGPT.
If you're processing personal data of other person, then you don't really have a choice in the matter: gain permission from them to transfer their data to a third party or self-host the model.