There are various studies that definitely prove that they are harmful in short and medium term (less than traditional cigarettes) and already has lead to death. There hasn't been a lot of research into long term effects as the trend is still new. So potential harm from long term usage is unknown.
Which studies? I'm legitimately curious as the only deaths I've heard of were because of the vitamin e acetate they were using in the knockoff cannabis pens.
The EVALI outbreak in US was determined to be caused by "illegal" products, but the materials to produce them are not banned in the US as far as I know.