I agree. It isn't even safe if a person isn't out to get you. I take a different approach to a Varia. There is no certainty in knowing if a vehicle in back of you will jeopardize you. I use the Varia to help me keep aware of things in FRONT of me. For example, if I were to make a left turn and the Varia alerts me that there is a vehicle in back, I won't even bother looking back. I would slow down and wait till the vehicle passes, consider taking the next exit or at times pull over and wait before moving over from the right side of the road. I still look back after there is no Varia alert. Or, there is a hill I ride up a lot. I get a Varia alert and wait to hear the vehicle tires hitting the dot bots on the left side which notifies me the driver is moving over. If I don't hear the tires hitting the dot bots, I would try and safely move over further to the right side. What I don't like about Varia alerts is when going down a hill and Varia alerts me there is a line of cars trying to pass. The only out is to let the drivers push you and try your best to maintain a safe distance/speed.Leaving the thread topic aside for awhile, I'd like to say the knowledge of the number of cars following my e-bike will not help if a driver wants to kill me, so the plain rear-view mirror and riding with my ears listening to the ambient shall do. (That is, I don't need the Varia; and I think any cyclist using headphones or ear-phones is somebody with a death wish).
One of the most dangerous things is when a driver overtakes you at the "envelope thickness distance": a Varia or a mirror cannot help there. Another dangerous manoeuvre is the left-turn (in the right hand traffic countries): either the driver following you understands what you are doing or you're dead seconds after.