Hammer Mastery gives +5% chance to hit for shatter. It is a great AOE attack and much easier to use than Greataxe’s Round-Swing. It also has more accuracy.
I don’t like the extra dmg to head unless I fight stuff like orc warriors that don’t drop their armor. All in all I much prefer Greathammers over Greataxes.
It’s better than round-swing, but worse than sword’s 3 space AOE attack. It’s better to use a sword if you are going for that, and axe if you want armor damage.
Hammer is supposed to be specialized in armor damage. As of now it feels neither specialized nor good enough to use. (talking about 2h)