Since a >100g collision will kill all the occupants anyway, catastrophic failure of chemical or hydraulic energy storage systems in those collisions isn't very important. But if there are any frame-distorting collisions that the occupants can survive, I wouldn't want the pressure vessel to be part of the frame.
Here's a tough question: If Ingo Valentin has been pursuing hydraulic hybrids for 25 years, where are his results? Has he built a 130mpg prototype or even a 50mpg one? Even a 20mpg prototype would be better than what we've seen so far.
His
1996 New York Times ad reports that as of 1996 he's recieved $750,000 in funding and needs $2.8m to complete a prototype. That just does not seem reasonable.
From where I'm sitting, Mr. Valentin appears to be more interested in funding and publicity than in completing the project. I have to question whether he's a reliable source of information.
His theory is sound, in theory, but can it be put in to practice?