There are credible reports of Catholics (Paul VI) wearing hair shirts and in Doctors of the Church (St. Theresa of Avila) engaging in self-flagellation. There's nothing new or secret to such acts of mortification. If anyone is doing serious harm to himself, it's most assuredly without the approval of the Church or the leadership of Opus Dei.
Self-flagellation originated in the ancient Jewish tradition, in fact they used to beat their chests with stones. There is a small trace of this practice still seen in the tratitional Catholic Latin Mass, where the congregation lightly tap their chests three times during the 'mea culpa', (through my fault, through my fault, through my grevious fault). But it's highly doubtful that any Catholic is engaging in an abusive form of self-flagellation, it's just another ridiculous story circulated for the benefit of the ignorant.
I think most people today would regard ANY self-flagellation as bizarre, even if it didn't cause physical harm. But some of the more extreme reports of Opus Dei members' activities include blood-spattered walls in rooms where they have been self-flagellating.