Personally, I think that a strong reiatsu is the Senbonzakura's greatest weakness.
My theory has to do with the fact that Senbonzakura is all about numbers and overwhelming the opponent. Each petal is given only a small amount of power like a splash of water, but combined together they make a tidal wave that washes everything away.

Also, notice how Ichigo was able to slice the blossoms away with his bankai speed? They were quick weak swings to knock the petals back.


Now, let's talk about the reiatsu. A strong reiatsu will make weak attacks have little to no effect. Remember when Ichigo was unable to cut Kenpachi with his sword? His attack had little heart in it so it wasn't very powerful, but the reiatsu reduced to nothing. Kenpachi is the king of strong reiatsu, I mean he has to handicap it with an eye-patch.


So this is my reasoning as to why a really really strong reiatsu could reduce the cherry blossoms to being ineffective. Because this is Byakuya's Shikai AND Bankai, it's significanly handicap him if he were to fight Kenpachi. He'd have to resort to that Angelic attack he had, but even then a fight of pure power like that would leaven Kenpachi advantaged.