Yea made a gfy of the Lucina getup, if anyone wants to break it down frame-by-frame:
http://gfycat.com/ScratchyLimitedBlackbuck
Gif is 30 fps so not exact, but the whole getup looks to take around 18-20 frames before you're free to act, though I'd assume there's a few frames of variability possible depending on character or how precise you time it. That's almost twice as fast as normal getups, which I believe are mostly in the realm of 32-35 frames.
So that's at the threshhold of being unreactable, or at least unpunishable on reaction with most options. Reaction time is average ~16 frames, factoring in 1 frame input delay I guess theoretically you could catch someone with a frame 1-3 jab or something. But there's also the fact that reaction times tend to increase a bit when there's multiple potential options to react to (this article is a good read about that btw::
http://ki.infil.net/reaction.html)
On top of this it's visually hard to distinguish from any other onstage getup option at the start, so realistically you wouldn't be reacting to the very beginning of the animation but instead a little bit after. As long as it's not used too predictably, this seems like it could be a huge deal purely based on how fast it is.