This was my first ebike mirror. I installed it and rode around the block. On my way back, I heard the tires of a car close behind. I studied the mirror and couldn't see it. The bike was too unstable for me to try to look back. I was relieved when we rounded a bend and it passed.
There were 4 reasons I had been unable to see the car. It was a convex mirror that reduced the image to 1/6. It was far from my face. It was around sundown on a heavily overcast day. The car was the same color as the pavement.
A mirror that can deceive you is worse than no mirror. I removed it as soon as I got home.
I bought the one below not realizing that it, too, was convex, reducing images to 1/3. It's supposed to be a left mirror, but I couldn't see past my arm. I mounted it upside down on the right and bought a right mirror for the left. That put them even farther from my eyes than the first mirror. The car is visible because it contrasts with its background, it's only a hundred feet away, and it's a bright day. I wanted better situational awareness.
This is the kind of mirror I'd been looking for all along: big, flat, on a stem that gets it out past my arm and up near my face, and adjustable while riding. Having a big mirror stick out may help discourage thoughtless drivers from passing dangerously close. It can get knocked out of kilter in the garage, either where it fastens to the clamp or at the ball joint, but it can easily be realigned while riding. What's more, this kind of mirror is cheap.
Below is another flat mirror. Two screws clamp it to the end of the handlebar, so it can quickly be removed. It, too, is easily knocked out of kilter and can as easily be readjusted. Being smaller and on a shorter stem, it vibrates more than the one above. I'll replace it with something bigger.
The one below is bigger. It's convex but reduces images to half, which I find tolerably large. I found it unacceptable because it can't be adjusted without a wrench.