This is around 5% of the energy density per kg of the NiMH battery in an Insight (kilowatt-second is a round-about way to say KJ). It can deliver that energy 20 times as fast as the Insight, which would make it attractive when highly efficient regenerative braking or rapid acceleration is required. It seems like a parallel hydraulic hybrid arrangement would be great for garbage trucks, city buses, and postal vehicles.
I require proper front/rear brakeforce distribution, and an outboard, cable-operated backup brake system on any car I own. I also need to be able to stop the car even if the battery / hydraulic accumulator is full. An inboard brake attached to a hydraulic motor can burn off extra energy for long descents and such, and conventional rear drums can be an emergency/parking brake.
The advantages of a tiny ICE in a parallel or series hybrid are plain, but one interesting limitation is hill climbing endurance. Once the battery runs out, you'll be climbing the hill on ICE alone, and 15HP won't propel a compact car very fast up a hill. Even 200lbs of hydraulic accumulator will only haul a Yaris-sized car up a 190ft ascent, assuming it starts at 100% state of charge. The electric hybrid's much higher energy density gives it an advantage here, where a stock Insight pack should be able to handle a 1000' ascent.