Hardness doesn't tell the whole story. Hardness brings brittleness and that is why it is avoided many times. Again, different properties for different applications.

There are plenty of WRC and F1 teams who are cryo treating gearbox parts for instance and are seing them last much longer between overhauls.

 

It works. Improves wear resistance for some carbon steels and cast iron ... example: trans gears and brake rotors. Also used for machine tools.