OOC

[DM Vitkyng]

Acctually the robot vulnerability to critical hits is sort of unique

See the Robot entry in the d20pfsrd here https://www.d20pfsrd.com/bestiary/monste...cts/robot/

Vulnerable to Critical Hits: Whenever a robot takes extra damage from a critical hit, it must make a DC 15 Fortitude save to avoid being stunned for 1 round. If it makes a successful saving throw, it is staggered for 1 round. The robot remains immune to other sources of the stunned condition.

So it doesn't actually cause x1.5 damage but can be stunned by a critical hit instead

Also, constructs.. which robots are.. not objects., in pathfinder do not get Hardness, though they may get Damage reduction. Only immobile objects get hardness. Usuallya construct made of a hard substance will have a suitable Damage Reduction ability