If you were to reference the anti-gun propaganda which went over mass shootings in recent history, Austin is distinctly lacking for a city of its size. Yes, there have been shootings. Charles Whitman is the famous one.
If you're saying that dividing cities into "safe" and "not safe" requires an arbitrary threshold, then yes, it's arbitrary where you set the threshold. But even then, you can non-arbitrarily label cities as "safer" and "less safe" when compared to each other.
But if you mean that the concept of "safe" is arbitrary: Safe means that there's a lower chance (not zero!) that I'll be on the receiving end of crime. Less chance that I'll be put in the hospital (or in the ground) or have my stuff stolen. That translates into less worry and less attention that I have to pay to keeping myself safe. (It does not mean that I can pay no attention!) That "less worry" and "less chance of being injured or killed" are not arbitrary. They make a real, concrete difference in my life.
I was more hinting at the idea that places that are "safe" for one group of people (often white middle-class people) are incredibly unsafe for other groups of people.
If safe/unsafe is dependant on who you are, then calling a place "safe" or even "safer" needs lots of provisos.
Anyway, bad choice of words. In retrospect "safe is a relative thing" gets closer to what I mean.