Prototype ultra(super)capacitor module builder and tester here. (Not an english grammar major ^_^)
This topic is pretty old now but I was reading through it and felt like chiming in a bit.
With several years of experience testing full modules, 6-36 cells, 1200f-3000f each cell, capactior banks, I have never seen them explode. Only ooze and leak when overcharged or shorted internally from miss wire. We could charge and discharge these things(full banks) at more than 100A without a problem. Balancing circuits were needed for each cell but the modules themselves were just simple 6 to 36 packs of coke can sized capacitors.
Our customers were heavy machines and buses and the these things took a beating a kept truckin. We vibe tested and dropped many modules and fail tests always exceeded our expectations.
The dangers came when these things VENTed from being overcharged. When the salt water solution inside boiled and vented, it created some gas which was cancerous on a very small scale so we still had to label that.(without boiling it wasnt cancerous) In an outdoor scenario apparently, no amount of the gas could "harm you" because it disperses quickly.
The biggest accident our customers had was with an entire shipping container full of caps. Maybe it was 2-4000V I cant remember but they wired the capacitors in such a way, without the supplied cables from us, that one module shorted and fried an entire rack of caps. Luckily no one was injured or died and they revamped safety measure after that.
The biggest non selling point was the weight for mobile applications. We started to fill them with thermal compound to disperse the heat and it made these things even more durable. However you'd break your back lifting them.
It was all good tech but the company had trouble reducing the cost lower than $38 per can if I remember.