It's a good rule of thumb to think about how you would fair running on the surface you're considering: if you think you would get hurt if you slipped or fell, it's probably not good to run SwiftPaws on it.
Also, excessive dirt, gravel, standing water, mud, debris, etc. might not be the most ideal surfaces to play on.
That said, if your back yard is more ground that lawn, you should be able to set up and run a course just fine. With careful planning, strategic placement of the pulleys to avoid loose dirt on turns for example and a willingness to learn by trial and error - you should be able to have a lot of fun with SwiftPaws.