I'll tell ya couple things about my ebay springer, and you can draw your own conclusion.
One, the hub mounting sucks. I had to make some axle locks to keep the axle in place and straight (see my thread in the bike builds section). Since it uses front-facing dropouts, the wheel could easily get away from the bike. I don't know electric hubs, but I guess they use a torque arm (like on a coaster brake) to keep the hub center from rotating? If so you'd have to figure out how to mount that.
Two, with a hub brake (drum in my case but whatever) the suspension moves noticeably with the brake. A 500W motor would have more than enough torque to do this, too. It's not dangerous, but it takes some getting used to.