Her ability seems to rely on identifying the target correctly. If she misidentifies it, the ability doesn't work. Perhaps it even needs a fundamental understanding of the target ie she needs to know the exact makeup of an alloy to destroy it (names don't matter) or to shapeshift, she needs to target the DNA. I wouldn't call that a flaw, but rather a very strict but intuitive requirement.I do like how her "Destruction" power while versatile does have flaws and faults.
I don't think some microbes can intercept her ability. Otherwise, she would have never thought of using the ability in the first place as it had never worked back then (there will always be microbes). I think she's misidentifying the shark - there's no guarantee that the shark she knew is the same shark in front of her. She might be thinking it a megaladon, which is extinct. How she calls it can't just be ignored.I think the reason she's failing at destroying the shark is because of the plankton in the water.
I think the idea of the destruction keyword is that she could dismantle the entire concept she destroyed, so that it could be reconstrued as something else. Destroying the concept of gravity allowed her to manipulate how gravity applies to herself, just like destroying DNA allowed her to recombine herself as a shark, I guess.How would "gravity destruction" make one sink?