While some RCs are capable of handling rough terrain, these are scale vehicles -- a pile of rocks is like a mountain to these cars, a water puddle is a lake. Encountering unexpected obstacles can cause damage by stopping your RC in its tracks, flipping it, cracking the body, or subjecting the suspension or other parts to excess strain.
And if there are water obstacles you plan to traverse, make sure you take proper water-proofing steps to protect you RC electronics and check the depth of that puddle or ditch -- unless your RC truck is also an amphibious vehicle you don't want to get in over your head (or roof).