Just like with leg power, road bikes are more aerodynamic and roll easier, so I would imagine a motor kit would push one along nice and fast. The frames also have lots of room for the frame-mounted motors. Skinny-tire Schwinns don't have much room between the chain stays, so getting a chain-drive setup might take some work. I'm sure people here can give lots of help.
The old mild steel Schwinn frames are strong. The higher-end chromoly ones should be also, but I was always too poor to own one. Here's my 1973 Continental with a homemade friction drive - still going strong.