I would rather run on a level surface even if it is harder. You can hurt your knee or ankle rather quickly running through some fields; but that's part of the experience I guess.
Running on an uneven running surface isn't bad for you at all. Most people run primarily on the roads - so if they encounter a long path with uneven terrain - the ancillary muscles that control stabilization generally tire more quickly. It's all about transitioning slowly and letting the muscles, that haven't been used very often, strengthen.