How does the Cryo Cannon's ‘Fragile’ upgrade work?
According to this video, the Fragile upgrade has a 50% chance on freeze to shatter regular enemies that are under 85% health, with an unknown chance to shatter Special Enemies that are below 100 Health
Edit: On personal testing, it seems like the chance is only calculated one time as soon as the enemy is frozen. Grunts usually lose ~15% of their health as you freeze them, so I've found this upgrade to be beneficial mostly when attacking large numbers of swarmers or grunts.