+- 10 ft-lbs off of 80 ft-lbs base is fine. However, when a repair shop takes an impact gun to lug nuts, they may well tighten them to 100 ft-lbs or more. That is not as good.
Over time one does develop a sense for when a bolt is sufficiently tightened. Personally I can't tell if a bolt is overtightened well. Maybe other people are better in this regard.
If lug nuts are not tightened enough, the wheel may eventually come off the vehicle. This is very bad.
If lug nuts are overtightened, the car is fine until the next service. Usually repair shops have powerful tools to take lug nuts off, and given that most cars don't have their wheels removed that frequently it's not an issue in practice. Problems may arise if the car is driven on salty roads and the fasteners seize together - then they are more difficult to remove, especially if overtightened.
In the event that a repair shop does break a lug nut, they can simply replace it along with the stud, if needed. The parts are cheap and the time saving on not torquing lug nuts to spec probably justifies the infrequent lug nut and/or stud replacement. Some of the shop may charge the customer for replacing these parts even though it's not really the customer's fault that shops tighten lug nuts with impact guns.
People who drive race cars, including amateurs, always tighten lug nuts by hand to torque specs because the wheels get removed much more frequently and the parts get a lot hotter than in regular street driving, and so the likelihood of problems if lug nuts are overtightened is much higher. Some people may choose to use non-factory torque specs but everyone I know tightens their lug nuts to their chosen specs by hand.