A crankshaft normally has a main bearing on each side of the crankpin, so the loads imposed by the piston are absorbed evenly on both sides. This makes the crankshaft what in structural analysis is called "simply supported", and is the most efficient shape: it achieves the required strength with the minimum weight of metal. That is called a full crank.
A half crank has only one main bearing, with the crankpin out to one side of it: it is a full crank cut in half at the crankpin. This is what in structural analysis is called "cantilevered", and unless you increase the size of the main bearing substantially, it gives much less strength, more deflection, and more vibration. Actually even when you do increase the size of the main bearing as much as is practical, it still gives all of those things. In structures, it is sometimes done because there is no alternative. In engines, it is only done because it is cheaper.
