If it is a quality battery and there are no cuts in the wrap or insulator I think the chances of battery failure are small. If it is a counterfeit or poor quality cell, or a badly designed or knock off mod, all bets are off. That is why we warn to buy name brand batteries from reliable vendors and watch for counterfeit devices. The vent holes won't in any way prevent a battery failure. They only provide a place for the very high pressure battery event to find its way out of the mod. If no such vents exists the pressure build up inside a closed metal box may blow it up like a grenade. Venting hot gas with some flame and the chemicals involved can definitely burn you quite badly, but an unvented metal mod could kill you or someone nearby with the force of metal shrapnel. I'm not saying it WOULD happen, I'm saying it could happen, and the risk is unnecessary.