Gaara's sand cushioned her fall and prevented damage. Otherwise she would be severely injured. She is lucky that she didn't take the full brunt of susanoo like Danzo.
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And both of these susanoos don't have as much power behind their punches as juubi tail so it's pretty weak argument to begin with.
Mei was shown with her guard when hit by both Susano'o and SS. Air palm is a pinpoint seeminly invisible attack which can target precise locations around her guard. If she gets hit while on the ground she is gonna get hit by tons of uprooted rocks while getting completely swallowed up like an ant by the impact alone.
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If Mei gets hit she's bound to get badly ragdolled and injured no matter what. She cant hope to properly guard nor land properly when hit by an attack that directly hits every section of her body while sending debris at it.
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If Mei uses water pillar then it blocks her PoV and Vacuum wall palm plows through it easily , still hits her while also taking her off her feet. Hiashi can either jump and launch another vacuum wall above Mei or simply launch a follow up blast unhindered.