Some two-cents:

As Fern has stated about negating powers of bestial types, how would the negation work? Just enough not to allow Rori to transform completely into a bird, or it's actually possible to reverse her back to human physiology?

Can powerful metas resist the negation? Like instead of completely switch off their powers, just enough to weaken them/prevent them from using their best moves? Will the others be able to resist the negation through, say, willpower? If we go with [@BlessedWrath]'s analogy of radio signals, there are still some ways to overcome jamming - it's not a foolproof thing.