Deeply Trivial finds evidence for why there is little evidence about the safety of e-cigarettes:
[T]he statistical sin here isn't really something the researchers have done (or didn't do). It's an impossibility created by confounds. How does one recruit people who have only smoked e-cigarettes or who at least have very little experience with regular cigarettes? What's happening here is really an issue of contamination - a threat to validity that occurs when the treatment of one group works its way into another group. Specifically, it's a threat to internal validity - the degree to which our study can show that our independent variable causes our dependent variable. In smoking research, internal validity is already lowered, because we can't randomly assign our independent variable. We can't assign certain people to smoke; that would be unethical. Years and years of correlational research into smoking has provided enough evidence that we now say "smoking causes cancer." But technically, we would need randomized controlled trials to say that definitively.
That's not to say I don't believe there is a causal link between smoking and negative health outcomes like cancer. But that the low level of internal validity has provided fuel for people with an agenda to push (i.e., people who have ties to the tobacco industry or who otherwise financially benefit from smoking). Are we going to see the same debate play out regarding e-cigarettes? Will we have to wait just as long for enough evidence to accrue before we can say something definitive about e-cigarettes?
For my part, their safety or lack of to the smoker makes little difference to me. I just don't like people blowing their exhaust fumes into my environment.