I own a uk legal e-bike (restricted to 15 mph maximum powered speed). !5 mph is about as fast as I would want to go on it in traffic or any kind of urban environment, where you might encounter cars or cyclists pulling out of side roads without looking, wayward pedestrians etc. It is quite exciting enough trying to bring the thing to a halt from 15mph in an "emergency stop" situation. I shudder to think what it would be like trying to brake hard from, say, 25mph...... If you don't hit the thing you are trying to avoid, you would very possibly lock the wheels up and end up on the deck anyway.
However, I do recognise that people who live out in the countryside, and who ride on more open roads, could give a faster bike the full beans.
I can only say that, for me, 15mph is about fast enough. Unless I'm going downhill and am having a rush of blood to the head. In such circumstances the other day my speedo read 38mph!