I have been working on Li-ion batteries for over 15 years. There are many cathode chemistries, cells designs, sizes, electrolytes...Some are safer than others. For instance, LiFePO4 is safer than LiNi0.8Co0.15Al0.05O2, and LiNi0.33Co0.33Mn0.33O2 has a good compromise of safety and performance. There are electrolyte additives that prevent ignition, separators that shut down.
Problem is, it is very difficult to pack a large amount of energy in a small volume and be totally safe at the same time. All it takes is a short, local heat elevation, and thermal runaway can happen. It is the battery pack design and electronics that can mitigate the effects too. And of course, the bigger the battery, the bigger the consequences.
In the case of Chevy Volt, fires have occured 1 week after a crash test. I say, this is still better than a gas tank exploding during the crash.
Cheap chinese Li-ion batteries have been known to be more dangerous than higher quality japanese or Korean ones. Quality control, cell homogeneity are key to build reliable packs. Hand made cells in chinese sweat shops, not so good...
In fact, I published the very first paper on the reactivity/safety of graphite anodes in Li-ion batteries., during my post-doc.