The Fatman in Fallout 3 is just overkill seeing as a 10mm pistol can kill anything in just a magazine or two unless its a death claw.
Then you install Broken Steel which eventually introduces variants of enemies that turn the difficulty curb from horizontal to straight vertical making the fatman the only weapon that stands a chance of actually doing any decent amount of damage.
Seriously, you can unload 90% of your ammo into one of these things and it does nothing and then your weapon breaks. What a giant load of crap.