I'm no expert on the production process, but I'm quite certain that good THC distillate is made just as natural and quality as high-end nugs themselves. It's just basically the THC extracted out of it. But like
@1LuvCane said, the problem is having confidence in where you are getting it from. I get most of my oil from the FL medical clinics, so I assume they've gone through proper testing. I don't believe any chemicals are added to legit THC distillate. But if you don't trust the source, I can appreciate the concern.
I am a huge vape proponent. I smoked cigarettes for almost 20 years and just couldn't kick the habit. Switched to vape (e-cig) almost four years ago and haven't had a single cig since. I have no interest in them anymore; in fact, they disgust me. I don't wheeze anymore, I don't cough up black sh-t, I don't lose breath quick, etc. I mean literally every ailment I had from cigarettes (and I could legit feel them killing me) is 100% gone with vaping. Now, does that mean it's "Healthy"? Of course not. Our lungs were designed to breathe air. But between e-cigs and regular cigarettes, there is absolutely no comparison in that regard. As even my doctor says, "nicotine doesn't give you cancer. The carcinogens in smoke do". I absolutely agree and am a living test case for how much better on your body it is.
So I look at THC oil vs smoking joints/bowls the same way. Plus, like I said above, they are just so much faster and cleaner and easier to use. If it were up to me, I'd be done with flower entirely. But I keep a stash for when I'm out of good oil.