User blog comment:Zathronas/Analysis by an author: Volume 2 episode 3/@comment-2006510-20140808072655

For the Ruby's Semblance bit, as we saw with Weiss and the many Glyphs she cast up the vertical cliff, casting a Semblance clearly takes some of your energy, so it isn't unlimited. Thus, I imagine having used it only for herself, Ruby's able to do it a lot - however, Penny was likely a lot of weight, being an android.

And as I also saw, she simply stopped too soon - if she'd done it a little longer, to right her balance, then reality + physics wouldn't have taken hold like they did (ie rolling her onto the street). Plus, anyone can be frozen in fear/surprise by sudden oncoming doom; it's the deer headlights issue.