Ok, so, a slingshot is a toy, not a weapon. However, you may be confusing it with a sling, which is actually an incredibly deadly weapon and definitely worth making the list of most underrated weapons.
A sling is a common farmer's weapon made from woven thatch and used primarily by shepherds to fight off wolves who are threatening their sheep. Those shepherds who regularly practice with a sling can be DEADLY accurate with the things. It was the weapon that David slew Goliath with. Some people are trying to argue Goliath probably had acromegaly and had trouble seeing and all that stuff in order to under-cut David's victory. But, no. The reason people feel the need to do that revisionist version is because they don't actually understand how deadly a sling actually is, mostly because they are confusing it with a slingshot which is a toy.
Yeah, a sling can very easily kill someone in one hit, you don't need some rare brain tumor condition in order to go down from a single hit by a sling. A sling can hurl a 1Kg rock straight at someone's head at over 60MpS. I don't care who you are and whether or not you are wearing a helmet, that WILL kill you. Plus, it has range. Primitive slinging cultures could consistently hit a target at 150 feet. They were freaking snipers with those things. That's hunting level accuracy, they could actually use those things to hunt and kill game.
I was harping on how underrated a warpick was before. Well, time to revise that. A sling is the most underrated weapon. It is so underrated that even though it appears in historical texts and the bible, and is a weapon available in D&D, it STILL gets underestimated and underrated by people. Getting underrated in the works in which it appears is the very definition of underrated, and definitely slots it a step beyond the warpick which is just not mentioned.
(video for info on just how effective the sling is as a weapon.)