"If the demon fox would've been sealed into Sakura, she would be the greatest of them all, as Kakashi has said so many times, she has the best chakra control. She would know how to use the Kyubi's powers.
Now."

Sakura only has good chakra control because she has so little chakra that she can't afford to waste it. Besides,
how great can you be when all you do is sit on your ass while Naruto and Sasuke do all of the fighting?