I honestly think that having to hold her breath is a great drawback, and that her lung capacity should just be that of a regular human. Which of course, she can train.

And there is a pretty obvious limit that she can have that no one seems to have touched on: how much time does it take for her to actually make herself intangible? I'd say about 0.5 seconds would be a good limit for that, and with reaction time, it could take her less than or over 1 second between seeing something she has to avoid, and actually activating her power.

In my opinion, thats pretty balanced.