Enchanted*, the hammer is a match for the Flamethrower in raw damage. In addition, it:
- doesn't need to engage, making it easier to beat enemies off allies
- can shove enemies away into irrelevance or as a pseudo evade
- is a Relic which helps with with things like Poltergeist
- benefits massively from Reliable and Enchant Weapon for reaching huge skill values for boss killing
- doesn't need to be reloaded, saving many actions and resources playing Extra Ammunition and the second copy of Flamethrower
The downside is that you do need to oversucceed to do 3 damage per hit, but skills like Daring and Overpower makes oversucceeding very reliable even before you start attaching Reliables and Enchanting it.
I've also found that the actions and resources saved from not needing to reload or replay weapons means that Guardians become more capable in other capacities, be it from healing or becoming able to pick up the odd clue with the excess resources. At higher difficulties the testless damage and +1 from a Beat Cop (2) was usually too much for me to give up, but the incredible reliability and relatively lower cost of the hammer means that it's easy to forgo the cop for other allies.
This is a particularly huge buff for Sister Mary, who struggled to compensate for her 3 in a class that revolves around monster slaying. Now she can swing a hammer along with the best of them.
@Susumu 's existing review, I'm more lukewarm on this weapon for Mystics. Mystics have so many good hand slot choices and just gained a few more. More importantly, Mystics also lack commits for the hammer to oversucceed at higher difficulties — you can't commit Guts to it — and don't have access to Enchant Weapon or Reliable. Therefore, there's less of an advantage over Shrivelling (5) (especially if you're only occassionally fighting), though it's nice to not have to worry about backlash and spell charges.
Still, there's probably an interesting build with Mind's Eye, allowing mystics to attack at double their , or to otherwise use the newly freed arcane slot for Seal of the Seventh Sign or Close the Circle.
* You could also enchant the Flamethrower, but the enchantment will be discarded if you need to play a new copy to reload it, and it's so overkill that it's impractical