What do you mean by comfortable? If you mean as easy to pedal, then the main difference is the added weight of the motor and batteries that creates the extra drag. The motor drag is pretty minimal.
If you mean comfortable as in riding comfort, then the big difference is the additional speed. You can get bounced around pretty good especially on the trails. I am actually seriously searching for a comfort bike right now. The K2 Easy Street Deuce seems to be the top runner at the moment. The main reasons for the upgrade are to obtain a higher gear ratio so I can pedal at higher speeds, and to save my back. I am already feeling some pain after only 40 miles.