Something moving that quickly through the atmosphere will most probably be massively decellerated by air friction. Saying that however on the basis that the artillery shell meets its target with said kinetic energy, it would be devestating yes, but you must remember that it is going to be falling vaguely vertically, and not exploding. Its force will be heavily centred on the pressure of the end of it hitting the ground and will probably make a rather impressive crater as it compresses mud buildings and the occasional unlucky person.
However just because something has the equivalent kinetic energy of a nuke, doesnt instantly mean it is as effective as one, an artillery shell such as that wouldnt be stopped by any amount of armor probably, but its blast is primarily vertical and towards earth.
By contrast a nuclear bombs burst is outwards and horizontal, this comes as a much more devestating explosion as the outward force will destroy buildings with a sidewards force (which they are much weaker against as they are designed to be resistant to the vertical forces of gravity ^_^) and the explosion is not majorly decellerated trying to pierce or compress mud and ground foundations, most of the destruction is focused on buildings, people etc etc.
Also nuclear bombs have an ionising radiation component which will primarily irradiate people and cause them to get early signs of cancer, go crispy and die quickly and all those other frankly disgusting things that come with using nuclear weapons, not only that, nuclear bombs have an EMP portion that will disable electronic equipment making it even more devestating again.
The artillery weapon may have some impressive kinetic energy, but what would you prefer, dropping a bus vertically on a few unlucky people or using the equivalent kinetic energy to ride the bus horizontally over land and run over a few even unlukier people?