The only reason it's always singled out as the most "op" weapon is because it's the easiest gun to tell you got killed by. The last word and red death are just as common but don't get nearly as much hate. I'm sure if they turned your screen green they would. Just my 2 cents.
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I believe the phrase is "Confirmation bias." Here's what I picture happening: Player enters Crucible: "I've heard about this hand cannon that causes your screen to flash green as it poisons you. I hope nobody uses it on me." Player gets killed any number of times by white/green/blue weapons, just shrugs it off: "Well, that's to be expected, I wasn't maneuvering or using cover and I pretty much ran straight at that entire fireteam..." Player encounters opponent with Thorn: "OMGITBURNSIMTRYINGTOSHOOTBACKINSTEADOFTAKINGCOVERWHYGODWHY--" GUARDIAN DOWN. "Yep, Thorn is -blam!-ing OP."