Has been used for 2 stroke motorcyle expansion chambers for a long, long time (50 years or so) but generally by competition departments and aftermarket suppliers, and of a far simpler type without a mold.
At base, do your calculations, cut two identical patterns from sheet steel (allow some sacrificial material onto which you braze/weld a union to suit a hydraulic pump, weld all round the edges. Fill pump with water, pump, and the steel will balloon. It's a nice way of forming fairly simple cone shapes, tube shapes, circles, etc. A £100 pump and a TIG welder is all that's required. Attempting it with compressed air is likely to result in serious injury.
More complex shapes either involve cut and shut or -- properly -- molds. Add the joys of TIG welding titanium and you're into a very different level indeed. `
But even at a simple level, hydroforming seems to offer some advantages over more conventional fabrication techniques. I fully intend to give it a go, someday.