If she absolutely phases out of our dimension, then the absence of matter would have to be filled in quite abruptly. She'd leave a vaccuum. And if she can phase out in the blink of an eye, there'd probably a minor crack like a whip. And there's the matter of appearing back where solid matter is. Fluid matter, such as gas and liquids, that would probably be okay. Phasing back in when solid matter is in the way would probably result in that structure plus your body being simultaneously destroyed. You can bullshit that there'd be a localized explosion, which would be an interesting last resort move. To go out with a bang

That isn't necessarily a weakness, just some details to the power that might be interesting.