Guys, this is new information to me. I was going on my experience working with a large number quality PCBs that lasted for decades. The vast majority of the PCBs I replaced were for upgrades and new features, not because the boards malfunctioned. So my experience has been that electronic components in general are reliable, as long as they're not mistreated (Voltage surges/spikes, leaking water, overloading, shorting. etc)
Of the small number of boards that did malfunction, I never learned the actual specific causes--even in the 70's we Americans (specifically USA-ans) had developed a throw-away-and-replace rather than a fix-it economy for anything less expensive than a washer/dryer. So I was unaware of the most common PCB component failures.
I'm going to interpret this as capacitors being the least reliable in a category of very reliable components: Electronic components with no moving parts.
I'd like to hear your opinions on why capacitors are so relatively unreliable. Is it an inherent problem, or is it a matter of manufacturers chronically under-specing capacitors, i.e., not providing enough margin in the specs for anything that the cap might be subjected to? Or maybe not enough voltage regulation within the circuits? I'm having trouble believing that capacitors fail without some preventable cause.
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Getting back to the original question: At present, supercaps just aren't compact enough to use in cell phones. You might look into using supercaps for a portable cell phone charger.