I'm on my 3rd brush hog. First 2 (Woods) had rubber guards front and metal skirts in the rear. Since I have mown my land dozens of times and hit nothing other than an occasional ant nest, I have deemed my land "safe". My current brush hog (Bush Hog) had no guards, and it will stay that way until someone throws a railroad spike across my fence and it kills me. Then I'll add guards for sure.
Kidding aside, if you know your land and there's no metal, sticks, and rocks, you're safe. If not, either walk the land and pick everything up, or add guards.