I don't think adequate studies have taken a look into the long term effects of all the solvents and oils they use aside from the nicotine. Intuitively, this just seems like a terrible idea putting non-water-soluble vapors into your lungs but I am definitely not a doctor.
It's not exactly a wild list. You can make your own mint vape liquid at home by combining 1 part vegetable glycerin with 1 part glycol, dissolving nicotine salt to the desired concentration (most dangerous part due to nicotine's toxicity, suggest getting it pre-dissolved in either one or both of the above), then dissolving menthol in to taste.
Don't get me wrong: it's not good for you, but it's a lot less bad for you than cigarettes, and it's not some great mystery as to what's in it.
