In a successful exercise program it's important to increase breathing and heart rates to enhance cardiovascular fitness.
Electric bikes have the potential to be very successful exercise machines for a simple reason.
They are more fun to use, so they are going to be used more. All electric bikes allow pedaling and many require pedaling. But it's an easy, power assisted pedaling. And a mild level of effort, over a period of time is a far better way to exercise than an intense effort for a short time. A very likely result is that electric bicycle riders will ride longer.