poor range. and the fact that they dont' do enough damage. 
a level 6 knight on earth patrol can take 2 shots from 331 shotgun from about 2 feet away. on most shotguns that's about half the clip. then you have a couple more before the 47 second reload animation.  yeah, try using one on a level 26 ROC strike. or a raid.  Good Luck.
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	Oh, I use shotguns pretty exclusively on Roc strikes. :) Works pretty great and I have fun there. But your points are all valid and I also think shotgun has so many negatives and positives aren't really that obvious.
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	Nailed it
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	i've used them in strikes and raids with great success. only stop using them when fighting bosses.
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	Yup, bosses have pretty much this oneshot kill melee. And shotguns lose their effectiveness from ranges other than melee.