Honestly, there's a degree of uncertainty over the long term effects, although for me, and many people I've come to know on ECF, there have been clear short-terms benefits, harm reduction compared to when we smoked, and the lack of some of the really bad chemicals from smoking removes some of the worst known long term negative effects from smoking. (I could still have long term things from my smoking decades, but the vaping isn't contributing new damage.)
Propylene glycol has had a fair amount of study, well documented, some of the work is directly applicable although not specific to vaping. VG/Glycerine I don't recall reading much on relative to inhalation over prolonged periods. The flavorings used are, in most cases, from the food arena, which means they've been eaten or drank but not inhaled, though some tobacco flavorings I use are from tobacco firms, designed to be applied to tobacco which is then smoked, to adjust the flavor. Colorings as a standalone additive I would never use (once in a while there will be a topic about if you can add colorings to get colored vapor). Nicotine at least for me I just accept, since I had been inhaling it for years a a smoker, but it is a mild stimulant and vaso-constrictor.
Some people do have sensitivities to PG, or VG, or various flavorings. These are real things, can be a PITA to trace down, to figure out what is okay for a person with sensitivities to use, however these sensitivities are the same as found with respect to use of lotions, or eating foods (not special to vaping).
I will accept the clear short-term harm reduction from using PV instead of smoking, acknowledging that the long-term effects are unknown, but likely far less than the multiple thousands of chemicals that we end up ingesting from burned tobacco. (the base tobacco doesn't have thousands of chemicals, many of them are produced from the burning of it) I know that my vaping introduces negligible TSNAs, the really horrible cancer causing chemicals of combusted tobacco, comparable to levels found in approved BP smoking cessation products (patch, gum).
Harm reduction, yes, but I certainly can't claim 100% harm removal/safety, just that it is a lot better than continued smoking.
Vaping hasn't been around long enough for any long term effects (or lack thereof) to be documented.
