I'm doing a test story myself right now and often the sims don't react simply because you have them focused on other tasks. If you let them play autonomously while the cheating or death happens you almost always get a reaction.. I'm doing death stuff now, and watching how the sadness goes, but I've done a few jealousy tests too :). Also if they are doing stuff they multitask at they often do pay attention. Traits can play in big time, for instance evil sims can get happy over negative emotions. Their moods also stack so that plays in too.
The best thing to do is not believe horror stories but try it for yourself.
