It's a general useful principle in life that nothing is hard to do once you know how. Some things require more practice and currency than others to feel comfortable.
This is a key theme. No one can be expected to be excellent at something the first time around, everyone is entitled to develop a skill, and thereafter expected to maintain it. Some 'planes require a bit more skill than other to land smoothly, but for any certified plane "normal" pilot skill will be adequate (whatever the FAA considers "normal").
Two important considerations are: Does the 'plane (by design) have enough control available for the pilot to maintain control, and, is the rate at which control might have to be applied within the normal observation and reaction time of a pilot? We would always expect a 'plane to have adequate control available, though with any taildragger, there comes a point where it is not possible to have enough control to correct a groundloop, but that's an exception. So the rate at which change could happen, for which pilot control input is required, becomes a prime point for consideration. What type of pilot does it take to stay ahead of the plane?
So if a certified 'plane is "hard to land", it's probably a relationship between the pilot's observation and reaction time, to the handling characteristics. As the 'plane is certified, it is understood to have met the standard of not requiring unusual pilot skill and attention, so, it's up to the pilot to manage the 'plane, rather than the 'plane to be branded "hard to land".