In a former life, I did scramjets for a living (NASP). The number one problem with scramjets is characteristic combustion time. If your vehicle is cruising at 3 km/sec and the vehicle's combuster is 3 meters long then the combustion time must be less than 0.001 sec. to get any thrust. Hydrocarbon fuel is chemically complex requiring many chemical reactions to break down to carbon dioxide. Hydrogen combustion is both simple and energenic. The big (huge) downside with hydrogen combustion is the low density of hydrogen fuel which requires a large volume fuel tank which means a large vehicle surface area that finally means lots of skin fricition.

Hydrocarbon scramjets work fine if you're staying below Mach 10, e.g. a self propelled armor penetrating round. However you must use hydrogen if you want to go into orbit. Unfortunately scramjets may not be practical for spacecraft propulsion.

Gary