E-bikes have become extremely popular here in Europe. My wife has one, a Dutch-commuter type from KTM. It gets her around our hilly area for leisurely riding and basic shopping and she is very happy with it.

Then you get those who race E-MTB's and who take it very seriously; very fit cyclists on e-bikes. Watts are watts - if you push out 200w on an unassisted bike and 200w on an assisted one, you are getting the same workout only with the e-bike, you go faster because of the additional free watts. So, yes, you can get a very good workout on one. Furthermore, here the bikes are limited to 15mph on electronic-assist, this is because of the density of cycle traffic in many European cities and so a speed limit is imposed to ensure e-bikers do not become motor-bikers in cycle lanes. But what this means for the cyclist seeking a workout is their assist runs out pretty quickly and so at above 15mph, they are doing all the work.

As mentioned by others, e-bikes have varying levels of assist and so workout intensity for those seeking more intensive training can get that too. But even for those who do keep a high-assist level, studies have shown that now they ride more, for longer. This equates to a fitter individual overall compared to when they rode unassisted bikes.

The lightest road e-bikes are coming in at under 23lbs now; Wilier's Filante being one.