Being able to tank and heal an injure are two very different things, the armor of the 4th makes him able to tank powerful attacks and s if sakura tanks the same attack she would get more wounds than the the raikage, because what sakura does is that she heals after getting hurt, the byakugo seal does not make her body stronger but makes her body to heal faster. So sakura first has to get wounded and survive the attack if she wants to heal from that, but if she dies before getting heal she cannot heal, for sakura to use her byakugo seal she has to survive the attack in the first place, as for the raikage he can tank a lot thanks to his lighning armor