There is the fact that there is no governing body regulating what goes into ejuice. Does leave one paranoid for a "Jungle" type situation where cheaper dilutants, dubious purity, and even additives that are highly dangerous, but produce a very desired effect (better vapor production, harder throat hit, etc).
Keep in mind this is true of any unregulated industry. Ever have bread cut with bone meal? Used to be common in this country (which is why the FDA was established, in part), and can still be found elsewhere. And bone meal is one of the nicer stretchers used... Sawdust, bleach, NaOH, anything white and tasteless. Heck, remember a few years back when Cumberland farms were caught diluting their milk with water?
Not that I have ever found "one" company that has done this so far. But it "could" happen. Maybe it has. Maybe some of those that write these horrible experiences are from bad batches.
And it wouldn't have to be an adulterant. Why, simple nicotine could be the problem. Its very toxic, and if you get the dilution wrong... That would be bad. Mistake a 38mg solution for an 16mg solution? Even worse, get an undiluted solution added to a batch that should of been diluted to 1/10? A simple manufacturing mistake could lead to a world of hurt.
This is why I am actually for the FDA regulating my ejuice. Requiring a label like all consumables. Testing certified companies for lead and mercury. Maybe doing some studies on safety. A good 20 year study to see the long term effects. Maybe a five year from a pool of initial non-smokers. Etc. You can't rush data. Heck, I still like reading 60 year and still going studies on aspirin.