The explosion and radius are just too much, shoot it inside any building and you could drop the entire building down (of course not in the game because even glass bottles are indestructible), I think they nerfed the Fatman in Fallout 4, but I never used one in that game so I can't say for sure.
In Fallout 3 if you are caught by the explosion, no matter how much HP you have, how high your level is or what kind of armor (DT and DR) you have (without cheating or something), you're dead.
Since it is a "lauching" weapon the range is not that big either, if used in real combat situations I can assume it would wipe out both friends and foes in a big radius, and would also facilitate accidents, like pointing it or dropping it on the ground and the projectile (Mini Nuke) drop face down on the floor and KABOOM! there goes half of your HQ or something

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