[oil pan 4]

If it stores seconds of energy then its only going to give that 25% boost in city driving. Then be limited exactly like electric hybrids on the highway with both likely see reduced fuel economy.

If it stores seconds of energy it would make a good postal delivery vehicle hybrid system.

Used hydraulics are cheap if you can find what you need but used electronics are widely available and dirt cheap.

How much do you know about hydraulics?
I find hydraulics to be incredibly expensive new and what you do find out there used is very limited and often leaky warn out junk. Just maintaining the hydraulic systems I work with is usually fairly difficult.
To get any amount of useable fluid quickly moved from an accumulator to a hydraulic motor you are going to need pretty large lines.
Anyone referring to hydraulics as being cheap or easy, well that is just an alien concept to me.

Also a hydraulic hybrid sounds like it would need a good amount of electronic controls and instrumentation to make it all cooperate as any kind of useable system. That is a whole entire other ball game. So now you have fluid power systems plus electronics If some one could put something like this together, chances are they wouldn't know squat about how to install and program all the controls with associated instrumentation. To me that would be the hard part.

I don't really see how hydraulics could be that much cheaper new. A hydrostatic motor about the size you would want to use to move a full size vehicle with any useable quickness would start at several thousand dollars. The ones I have experience with were diesel engine driven and moved munitions handling trucks weighing between 3600 to 8000 pounds empty, both used the same hydrostatic drive and these hydrostatic motors just by them selves cost over $5,000.

I think the University proved that building a hydraulic hybrid makes for a great academic exercise and not much else.
That is why you don't see all kinds of hobbies and experimenters building them, they almost all go electric.