Rounds like 5.7x28 are one of the reasons everyone wears plates now. (All that M855 and M855A1 out there is the other biggie)
In a world where you don't know what armor they are wearing, and have the civi versions of 5.7, I really wouldn't expect armor penetration. It may happen, but plan for it not to.
There are a number of reasons this carbine wouldn't be my SHTF choice, but the biggest is at the price of 5.7 I can't practice enough to maintain proficiency. I think, in general, a rifle that takes a wierd round, in it's own magazine, with uncommon proprietary wear parts is a particularly bad choice for a situation where modern logistics has broken down, regardless of the performance of that round. YMM, of course, V.