It takes Chakra control to an extreme level and that is why Naruto struggled to acuqire it because he sucks at controlling his chakra. Sakura releases the precise amount of chakra from here fists thus smashing walls and stuff. I don't think it has something to do with having a wind affinity because Naruto has a wind rasengan which is different from the regular rasengan. I wouldn't say she doesn't have enough chakra because it can't be too low if she can smash shit up . So why can't Sakura or Tsunade acquire rasengan. Tsunade has enough chakra to summon that huge ass snail so even if Sakura didn't have enough, Tsunade should definitely be able to do it. Comment.