They can be very harmful. In fact a famous early American colonist was stung by one and his crew thought he was dying and even dug a grave for him.
It really SUCKS when you are fishing for whiting with four pound test a big school of these guys come cruising by and all of your rods start screaming. haha
Yeah, but so can a lot of animals. I dont think it's fair, especially when talking to general public, to sum up any animal as "harmful".
And I love catching these guys :grinyes