The Raspberry Pi does not perform a safe automatic shutdown when a power failure occurs, the supercapacitors will be drained and the Raspberry Pi crashes.
The reason is an internal pull-down resistor in the Raspberry Pi. This weak internal resistor works against a 10k pull-up resistor on the Juice4halt board. Even when disabling the pull-down by a command (e.g. "gpio –g mode 25 tri") and hence raising the voltage to a 3V level, the pull-down will be activated again at the next power up.
There is a transistor in series with the internal 10k resistor making the 10k pull-up not linear. The consequence is that pulling high is more strong at voltage levels below 2.1V than at voltage levels above 2.1V. Despite a 2.1V level, the Juice4halt is capable of safely interpreting it as a high level.
The Raspberry Pi's internal pull-down resistors are enabled by default. Turning the pull-down to a pull-up on GPIO22 (e.g. "gpio -g mode 22 up") will fix the problem.