One is hardness. Materials that are heat treated to higher Rockwell numbers also develop higher ultimate tensile. Which would lead you to conclude that more is better. Well the answer is simply NOT always. Tensile is only part of the story.
All materials react differently at higher hardness levels.
Some become very brittle, greatly reducing their fatigue life. Others may develop stress corrosion which is a special form of hydrogen embrittlement in which the material is attacked while under stress. This occurs when the hydrogen in the air we breathe penetrates the material. It's as simple as improper handling. Moisture from your hands can deposit minute amounts of salt and acid on the surface of the material which starts the corrosion process.