I have a bike with an electric motor front wheel. Bought the wheel kit, got a battery for it and put it all together with a friend. It's great. To your points:
1. Max range for mine if I don't pedal is around 20 miles, but I pedal a lot because I enjoy it, so I rarely get down to under half way on the battery (bottle type).
2. It's fine in rain. You're wet, which is less than nice, but the bike is fine.
3. Don't expect massive speeds, especially if like me you're a bit tubby (OK a lot tubby). But then again I'm not in a rush, and enjoy cycling a lot in better weather. Not so much in bad weather.
4. Don't know about cargo bikes, I just carry me, a backpack and maybe some shopping now and then. Never noticed any real difference in performance in those scenarios.
5. The main thing I found with the electric motor is it's really made me more adventurous on a bike. Since I'm getting on a bit, the idea of doing 20 miles round trip was kind of scary, but with the motor, I'm happy to cycle off on a 10 mile jaunt into town, knowing that if I meet any headwinds and long hills on the way back I've got a faithful friend to help out. :)
That's about it really.