Sure they may store my personal data and I would prefer if they didn't sell it to any third party. I would also prefer a product which can store information locally on the phone but never sync it to the "cloud".
The thing is, there is no such products because people are stupid and would be furious if they switch phones and their data was lost on them because they didn't understand that the data was only stored locally and never left the device.
I would buy such a product, if it existed, but it don't AFAIK. Fitbit stored the same amount of data as Withings do, it's just because Google bought them that people are worried.
Withings is basically a rival to Fitbit and since you don't want the data to ever leave your wrist, this sort of product is clearly not for you.
