How dangerous are these batteries really?, most of us have...

How dangerous are these batteries really?, most of us have numerous Li battery type driven devices and gadgets, watches through to power tools, with many more to be invented and consumed.
I can only recall one potentially dangerous home situation when my teenage son decided he was a mobile phone repairer and punctured an iphone battery with a rather large screwdriver (picture walnut and sledge hammer), it proceeded to swell up like a balloon after some time, but no flames only a noxious smell.
On the other hand during his petrol lawnmower mechanic days he managed to burn down half of our shed with just a spark !