Reading some of the responses I would sat it's all relative. You could use a paring chisel, as was mentioned, to chop a mortice with out any trouble... But if you want to chop many and do it all efficiently you will need a pig sticker. In the days that pig stickers were popular the craftsmen were required to chop mortices at an amazingly fast rate and I suspect they were hitting and cranking on those mortise chisels much harder than he was in the video. Now, we chop mortices because we want to experience the neander way and time and the ultimate robustness of the chisel isn't nearly as much a factor anymore.
Sent from the bathtub on my Samsung Galaxy(C)S5 with waterproof Lifeproof Case(C), and spell check turned off!