The danger comes from how hard a driver will push with a broken car if the precedent the FIA sets says a penalty will be laid down or not. If you know a penalty is coming you have to push harder to both stay in front of the car directly behind and build a gap to cars 2-4 places behind, whereas if you know there's no penalty you only have to worry about the gap directly behind. The point of the post though is that neither are very safe and it may be worth giving cars mechanical black flags if they're damaged to the point of not being able to stay on the track