While groundhogs are not typically known to attack humans, if they feel particularly threatened or are protecting their young, they might attack. Injuries caused by groundhogs usually aren't severe, but they can transmit bacteria and diseases. Groundhogs can carry the rabies virus in addition to several other illnesses like tularemia. These animals also transport fleas and ticks into your yard, which can cause further pest infestations.
If groundhogs take up residence on your property, they do present a threat, thanks to their continually growing front teeth. They need to sharpen these teeth and grind them down and will gnaw at things constantly to keep them from overgrowing; this can result in severe property destruction.