> Batteries are an energy store. If they go wrong, they are a bomb like any other large energy store.
It doesn't matter how much total energy they have stored, it matters how QUICKLY they can put out a large amount of that energy. A block of wood has more energy than your fireworks example, but one goes boom while the other slowly radiates heat. Shorting-out a low-energy-density super-capacitor is much more exciting than shorting out a high-energy battery...
Except with batteries, it's almost always the reactivity of the chemicals that gets you. Lead-acid batteries are actually a very tolerant of extreme abuse, EXCEPT for the fact that they generate hydrogen and oxygen, which is explosive. Similarly, Li-Ion polymer batteries would only get hot when shorted out, except for the fact than their ingredients combust into an impressive fireball once ejected from the casing by a fault condition.
Ni-MH batteries have about 2/3rd the energy density of Li-Ion polymer batteries by volume, but they're incredibly stable and in fault conditions just get hot and simply don't cause the conflagrations Li-Ion polymer batteries do. LiFePO4 chemistry Li-Ion batteries are similarly quite a bit more stable than Li-Po.