Required Secondary Powers dictate she knows when to stop :P

Also, I explained in the drawbacks why it's nowhere as convenient as it first looks (not that I'm implying you didn't read it or anything). Her ability to interact with her surroundings when traveling at light speed is incredibly minimal due to being massless, and her abilities are practically useless for offense unless she feels like dropping herself from somewhere high up when she's super dense, which is both easily avoidable and likely to send her crashing down through the mantle like a bullet. Her super durability can be maintained for like five seconds at most before she needs a few minutes to a few hours to let her heart recover from being stopped for so long, and objects are only weightless when she's carrying them, meaning it's great for stealing stuff (which is what she does), but not so great for anything else. 

Basically when she's confronted with adversity her only real option is to book it. Great for stealing stuff, not so great for superbrawls.