The scraper will follow the undulations but can make a washboard along the belly. You can control this somewhat by alternating the angle the scraper is pulled along the surface. I go one scrape with the scraper perpendicular to the belly, than at a 45 deg angle to the left then the same to the right. By angling the scraper at a 45 you take down the high spots to help control the washboard.
You will have to pay more attention with undulating surface than you would with a flat surface.