Dave and Kathy,
Sweepers are designed to be durable, long lasting, and simple. Think broom bristles on the wheel of an axle. That means these sweepers will work great in many conditions, and not so great in others. If your lawn is very uneven in several areas, you will see patches where the broom has missed or skipped.
If there are a unlevel areas you may be fine, but a very hilly lawn may cause quite the issue.