, since you posed the question most directly, this concerned me as well. Fortunately, terahertz radiation has a much lower photon energy than X-rays or other kinds of EM radiation (considering its placement on the spectrum), suggesting that its effects are most likely "...thermal in nature and, therefore, predictable by conventional thermal models (http://en.wikipedia.org/wiki/Terahertz_radiation#Safety)
." But, that's obviously a prediction only. I suggest reading the rest of this Wikipedia page for more information; people have speculated that it might cause "bubbles" in DNA by providing the right kind and amount of energy to "unzip" DNA. This would be bad, but the article suggests that these ideas, developed through simulations, are likely implausible in realistic settings.
My first question whenever anyone talks about harnessing new parts of the EM spectrum is the same as yours: are people certain that it's safe to toss around a bunch of these waves? But I think the response to this question should also definitely be: let's find out, and exploit the heck out of it if possible! We'll see with time if this technology becomes feasible.