They are certainly in any way identical, but to engine break, you usually need to switch into a lower gear (or a couple gears lower), which means that the clutch needs to reattach while the engine is rolling at a higher rpm than the gearbox can handle in that gear. Every downshift will do another "reattachment" through the clutch.

Which means that there will be some friction braking before internal engine resistance "takes over". Now, while both brake pads and clutches use "similar" material, it's not the same, and the cost is not the same (clutches are usually more expensive in modern cars).