That they are safer than cigarettes is about the only thing we've been able to honestly say and I think that kinda covers it.
Vaping is harm reduction, not harm free.
Keep in mind that,
"safer than cigarettes" sets the bar pretty low. 
I think that is a rather significant point, at least, it is for me. As there really aren't THAT many things you can do to your body that are as bad as what cigarettes do.
Safer than cigarettes doesn't mean safe. What is speculation is how much safer does something have to be in order NOT to cause a disease process?
Is "safer than cigarettes" any kind of guarantee that a disease process will not be set in motion by engaging in that behavior? NO.
Safer than cigarettes and harm reduction. That is where we're at, and have always been at.
Either of those things
can be increased or decreased though, depending on the flavorings used, the manner in which people vape, materials of delivery devices, etc. We just don't know yet! Everyone should just go with their instinct. I do. (less is more for me across the board.)
I am of the opinion that, as Dr. F and others have noted, eliquid probably shouldn't have ingredients that may represent an increased inhalation risks......that would seem to increase the possible harm.
Again, use your own instinct as guide, absent real studies in large populations and long term.