Likely != some
Of course some batteries explode. Shall we also ban laptops and cell phones?
As for content of the juice, sure there has been a few incidents of chemicals discovered to be potentially unsafe, but that's a far cry from " tend to produce some sort of unexpectedly toxic chemical". (What does that even mean, it sounds like we live in pre-scientific times and the world is a completely mystery to us.)
> How can you be sure that some fluid works fine with most e-cigs but when used by one that runs a bit hotter it doesn't cause a fire or have some chemical reaction between components that generates some dangerous gas?
There is no theoretical basis in chemistry to believe this, and there is extremely widespread usage of ecigs with no incidents.
But sure, anything in the world is possible I suppose considering you cannot prove a negative. I'm going to start wearing safety goggles when using my computer in case the screen explodes, after all, even though there's no theoretical reason to fear this and no incidences of it happening, it could, couldn't it. :)