It looks like it's more aggressive than boxing, so are the long term injuries, because boxing athletes dodge many punches, while in slapping all the hits literally fully hit the adversary's face without any chance of minimizing the damage received. Slapping is mostly about resistance and strength. Boxing involves some other skills like dodge that are always useful when facing a big challenge.
Due to this fact a weaker boxing player can overcome a most powerful one if he uses a good strategy. The same doesn't apply to slapping, so it's also a more obvious game, easier to predict the results.