An allegory is a literary technique that is somewhat like an extended metaphor and the goal of an allegory is to present a complex social problem by figurative meaning in order to grasp the full extent of the problem. A famous example of this can be the allegory of the cave as created by Plato where the idea of understanding the true nature of world is presented through people sitting in a dark cave. When they leave the cave, they are blinded by the sun but eventually they learn to see the world properly. He used this allegory because he believed that common people who don't work on themselves and only take things without thinking about them is similar to sitting in a dark cave and that illumination can be difficult but ultimately achieved.
Ambiguity is a literary technique that is used in order to present a plethora of different meanings without any meaning being the true one. It is great for trying to better explain issues that are not clearly presented as black and white. An example of this might be a debate on ethics and the nature of morality. Ambiguity can be used here because it is highly difficult to precisely and in a normative way to present what is good and what is bad and to describe why anything might be or might not be good or bad.
