Super Capacitor battery replacement, Lead Acid vs Capacitor explosion
It's pretty much the same, the explosion is faster because as soon as there is an internal short all the energy in the capacitor is dissipated into heat almost instantly. As far as gasses go, it depends on the electrolyte, and usually it's hydrogen. If the dielectric starts on fire, the smoke could be toxic.

Capacitors don't have near the storage capacity that batteries have and most capacitors leak current faster than batteries.