The trainers on first aid courses I've attended have all said similar things. Yes, you are quite likely to break a casualty's ribs if you have to give them chest compressions. However, if you're a trained first aider and you're considering giving them chest compressions in the first place, presumably it's because you have ascertained that their heart isn't working properly and their life is therefore in immediate danger. Unless giving the compressions is getting in the way of something more important, like calling professional help as soon as possible or using an AED if one is available, it's practically impossible to make things worse for the casualty by trying.