Pros for Supernova Theory
quite capable of causing the devastation needed for a mass extinction.
Cons for Supernova Theory
the absence of Pu-244.
cannot explain why some organisms would have survived and other perished.
cannot explain why marine organisms would have perished. Oceans are highly effective screens for radiation. It would have needed enormous amounts of radiation to penetrate deep into the oceans to exterminate marine life. With that much radiation, nothing on land would have been able to survive.
supernova would have to be extremely close to Earth (in astronomical terms). Statistically, the cards are stacked against this.
if the cause was a supernova, then devastation should have been much more intense on land.
the increased radiation would have tainted if not destroyed our atmosphere. Theoretically, the planet would not have been able to recover by now.
Note: The theory was disproven using isotopic studies. Along with Ir being created in a supernova, there is also Plutonium-244 created (a relatively short lived isotope of plutonium). Pu-244 is not found on the Earth and if found on our planet, its source would definitely be cosmic. Analysis of the boundary layer clay sediments show no signs of Pu-244. If the Ir did need come from a supernova then it should have been accompanied with Pu-244. Therefore the theory has been disproven. Some scientists argue that this alone is not enough proof; and there is not enough data or knowledge about supernovas to conclusively prove or disprove this theory. While others argue, the absence of Pu-244 is conclusive enough. As is always the case in Geology, the debate rages on.