In general, my thinking is that it's probably not quite as safe as breathing fresh air, but considerably safer than cigarettes. And that's probably still true even if one particular ingredient turns out to be carcinogenic.
I guess if I were a pessimist, I would think back to the Kent Micronite filter which, for a time (long before my time, actually) contained asbestos. It turns out you would have been better off sticking with the unfiltered cigs. So if there's something along those lines, I guess I'm willing to take my chances. It doesn't seem particularly likely.
It seems to me slightly more likely that a particular batch of e-liquid will turn out to contain some sort of toxic chemical as a result of a mix-up at the factory. In other words, when the employee reached for the bottle of PG, they actually grabbed the bleach. Or possibly more likely, the mixed up a batch with a dangerously high level of nicotine.
Again, I'm willing to take my chances with those as well. After all, there could be a similar mix-up with any kind of food or beverage product, and I don't spend much time worrying about those possibilities. I have started buying my e-juice in advance, which provides me some protection. Presumably, if there's a tainted batch floating around, I would eventually hear about it before I opened mine. Also, I do tend to add the new bottle to what I've already been using, so if one bottle has a potential nicotine overdose waiting for me, at least I'll be diluting it, and would probably notice the problem.
So I guess it's prudent to consider the risks, but not allow yourself to be paralyzed by them.