[According to Marla Spivak, they had trouble killing capped brood with dry ice consistently when trying to come up with simple ways to do tests for hygienic behavior.]
This is mis-characterized for the question at hand.
When testing for hygienic behavior is VERY important to chill brood to the point of death, without over freezing them (frost bursting the larvae) The difference is that a deep freeze will result in a 'mortuary response', not a 'hygienic response'.
It isn't difficult to kill capped brood, its doing so with consistant and controlled results to a hygienic threshold that takes skill.
My experience is that much depends on the colonie's instinct to cluster, and thus abandon edge brood.
This threshhold tends to vary on a number of factors (actual temp, draft, species, ambient heat from adjacent brood, and colony critical mass).
One must be careful not to assume that capped brood is necessarily hardy to cool or cold temperatures.
Alive and emerged does always imply healthy. Many of the critical stages of development occur while capped (wing/eye development). And if these stages are impaired, may reduce the life span or productivity (impaired eyes in mating drones).
I would never assert, just because they are capped, or even emerged that they are/were 'bullet-proof'.