A pasture with hills, gullies, rocks, logs and other natural obstacles will do nothing but good for a horse. It toughens their feet, builds up muscles they don't get in a flat pasture, helps maintain good conformation, and teaches them sturdiness and surefootedness. The pony I am getting soon has lived in a paddock for a few years now - The whole paddock is a steep hill covered with rocks, trees, stumps, logs, and yep probably rabbit holes. When she was put in her slight cow-hocked-ness fixed itself, and she now has great muscle tone from going up and down the hill and between the rocks.
Horses can hurt themselves/chipfeet etc. anywhere. If they are going to do it, they will find a way no matter how safe the pasture.