They are power, life and reliability.
The power is about the capability to accept or discharge electricity instantaneously. Many familiar battery applications such as car engine starting or wireless transmissions are actually about power. It matters less how much energy stored in the battery; but rather how fast it can be discharged. The ultracapacitor is more than 10 times better in power than the battery devices. Power where it is critical to the application commands a high premium.
Rechargeable batteries last less than 1000 full-cycles. The ultracapacitor last up to one million cycles. Battery-based applications needs many replacements and expensive servicing; on the other hand, ultracapacitors in a hybrid electric transit bus last beyond the lifetime of the application(15 years and 1 million miles).
The corollary of semi-permanent life and the wide operating temperature band. means high reliabilityand no maintenanceeven under extreme environmental conditions. In contrast, batteries are fragile devices requiring constant monitoring, protective circuits and thermal management which translates into a lots of $$$.