To summarize a lot of details, when done right, the cropped Delta wings of Thunder and Viper make for a highly maneuverable fighter at both high and low configurations, and very good consistent turn rate. Large Delta wings make for excellent instantaneous turn rate and provide good performance in thin atmosphere at high altitudes. The also have excellent climb rate. But their large surface area causes drag at low altitude causing altitude to drop during the turn. Cannards are a way of overcoming these problems. Think of them as equivalent to the horizontal stabilizer tails on the empennage, except they provide lift at the front, thus providing lift rather than stability. They can mess with the airflow of the main wings hence they need to be designed very carefully. In the end you live with a compromise and a complicated design. The Viper is a shining example of simple is powerful.