The only realistic risk so far is addiction and a nicotine addiction doesn’t ruin lives. Other than that it’s marginally bad for the heart and so far atleast not carcinogenic.
> Nicotine in tobacco can form carcinogenic tobacco-specific nitrosamines through a nitrosation reaction. This occurs mostly in the curing and processing of tobacco. However, nicotine in the mouth and stomach can react to form N-nitrosonornicotine, a known type 1 carcinogen, suggesting that consumption of non-tobacco forms of nicotine may still play a role in carcinogenesis
Cancer risk is more complex than just carcinogens. Nicotine is known to promote the growth of existing cancer cells, and in multiple ways. A big thing with cancer that not many people are aware of is that we all have cancer cells, and get new cancer cells all the time — but that the human immune system is normally effective at detecting and killing them before they have multiplied too much. Old cancer mutations can lay dormant or kept in check for many years, but if promoted and/or the immune system gets stressed or suppressed, they'd grow and you'd "get cancer".
Different types of E-juice also contain additives for flavour, and we still don't yet know the long-term effects of some of those — when ingested as vape — which is a different to being swallowed. And by long-term, I mean 20 years or more, which in some cases is the time a cancer cell can take from formation to detectable tumour.
Presence of the Carcinogen N′-Nitrosonornicotine in Saliva of E-cigarette Users: <https://pubs.acs.org/doi/10.1021/acs.chemrestox.8b00089>