Shattering into tiny pieces is the worst feeling. I always thought any heavy tool would work for cracking rocks open. Seeing that mess on my workbench made me finally buy the right hammer. Now I keep a proper geology hammer and safety goggles in my bag, no exceptions.
That "shattering into tiny pieces" line is key. Sometimes you want gravel, not a clean split, and the wrong tool makes it an accident instead of a plan. Your proper hammer probably gives you control over which result you get.