I can only see 3 cases for a bicycle powered by a motor:
1. you're physically disabled and can't pedal without a motor.
2. you use it in place of a moped for basic transportation to work, etc., not fitness.
3. you're a racer and want to cheat.
The beauty of an actual bicycle is you can pedal at your own pace and level of fitness. If you lack balance, get a 3-wheel version. Not that fit? Fine, just pedal slower in an easier gear and you'll still improve fitness. Why the need to go fast?
I maintain a couple of e-bikes for some elderly neighbors so I've ridden them many times. They are poor-handling, ungodly heavy and faster than the skill level of 95% of their riders. Every time they break down, somebody has to go get them because they're too unwieldly with a dead battery. Ridiculous for an able person.
< rant mode off >