The hammer doesn't always drop - half the time I'd say. You can always reload but it is quite weak anyway.
If the drop rate isn't 100%, it's pretty lame.
Yeah, not sure why a unique wouldn't have a 100% drop rate. At best, I could see getting a unique guaranteed in certain situations, but not a particular one (e.g., a box of treasure that might drop a unique belt or a unique ring.)
