i have had that happen with grinders a few times. once last week that just tore through my new jeans. the other times it was with a cutting wheel and i dropped the grinder. it didnt cause any real bleeding just kinda some road rash type abrasion. but im the kinda guy that welds in shorts and a t-shirt with no gloves. so i dont find them unsafe as long as you dont use a wheel that you have dropped and is all messed up.
at the ol body shop we had a guy using a cheap cutting wheel that had been damaged and it flew right off the grinder and kinda stuck in his leg. i cant remember if he needed stitches or not but he did go to the E.R.