Privacy is not even the worst aspect of these smart trackers. The band is always on your wrist emitting 2.45 GHz RF signal 24x7

The problem is the proximity. 

It’s ALWAYS on the body. 

 good point 

 There are atleast 2,000 studies that show harm from non-ionising radiation ranging from anxiety, headaches to cancer 

 Yes, but you can turn off BT and Wifi and only use them when syncing.  

 True. 

Oura is notorious for not turning off the wireless even when in airplane mode