Electric Bikes

Well, first there are different kinds of electric bicycles, then there are different goals, and then there are different reference frames.
Where I live, electric bicycles are only allowed to be electric ASSIST bicycles to be legal. Meaning that the motor isn't allowed to supply any power UNLESS the rider is ALSO pedalling. And the motor must CUT power at and above 23 km/h.
On such a bike, a ride will offer some degree of exercise, particularly if the comparison is to travel by car, bus or train.
But - of course - usually not as much exercise as an unassisted ride.
Then there are electric bicycles that aren't assist bikes, but rather electrical mopeds/scooters. Riding one of those will only offer marginal exercise benefit.

Not really. Electric bikes are made for people whose commutes include hills that would otherwise be exhausting or impossible to bike, or who are carrying lots of things that make the total weight very difficult without help. The electric aspect gives you the ability to get up a lengthy hill without working nearly so hard.

I wouldn't recommend an electric bike for training. Honestly, the only people that I've noticed riding electric bikes are pizza delivery guys and middle aged men who have a bit of distance to travel. If you're young and you want to train properly, I really recommend a bike without any motor assistance. Also, keep in mind that the motor also adds extra weight to the bike that you don't really want.

Hmm, I wouldn't say they are the best of choice for training.

To me they feel like a beat of a cheat because you've got that knowledge that you can switch to electric power at any time and when your really pushing hard, that temptation becomes all the more.

Unless there are significant circumstances, I would say get a normal road, hybrid or mountain bike.